<PAGE>   1
                                                                 EXHIBIT 10.13.3

                       FIRST AMENDMENT AND SUPPLEMENT TO
                                CREDIT AGREEMENT

                                  BY AND AMONG

                              REXENE CORPORATION,
                                  AS BORROWER,


                            THE BANK OF NOVA SCOTIA,
                                   AS AGENT,


                                      AND


                          THE LENDERS SIGNATORY HERETO



               EFFECTIVE AS OF THE FIRST AMENDMENT EFFECTIVE DATE
<PAGE>   2


                       FIRST AMENDMENT AND SUPPLEMENT TO
                                CREDIT AGREEMENT

        This FIRST AMENDMENT AND SUPPLEMENT TO CREDIT AGREEMENT (this "First
Amendment") dated effective as of the "First Amendment Effective Date" (as
defined in the hereinafter described Credit Agreement, and herein sometimes
called the "Effective Date"), is by and among REXENE CORPORATION, a Delaware
corporation ("Borrower"), and THE BANK OF NOVA SCOTIA (in its individual
capacity, "Scotiabank"), as agent (in such capacity, "Agent") for each of the
lenders that is a signatory hereto or which becomes a signatory hereto and to
the Credit Agreement as provided in Section 12.06 of the Credit Agreement
(individually, together with it successors and assigns, "Lender" and
collectively, "Lenders").

                             W I T N E S S E T H :

        WHEREAS, Borrower, Agent and Lenders are parties to that certain
Amended and Restated Credit Agreement dated as of April 24, 1996 (the "Credit
Agreement"), pursuant to which Lenders agreed to make loans to and extensions
of credit on behalf of Borrower; and

        WHEREAS, Borrower, Agent and Lenders desire to amend the Credit
Agreement in the particulars hereinafter provided;

        NOW, THEREFORE, in consideration of the promises and the mutual
covenants herein contained, the parties hereto agree as follows:

                             ARTICLE I. DEFINITIONS

        Section 1.01 Terms Defined Above. As used in this First Amendment, each
of the terms "Agent," "Borrower," "Credit Agreement," "First Amendment,"
"Scotiabank," "Lender," and "Lenders" shall have the meaning assigned to such
term hereinabove.

        Section 1.02 Terms Defined in Credit Agreement. Each term defined in
the Credit Agreement and used herein without definition shall have the meaning
assigned to such term in the Credit Agreement, unless expressly provided to the 
contrary.

        Section 1.03 Other Definitional Provisions.

              (a)  The words "hereby," "herein," "hereinafter," "hereof",
       "hereto" and "hereunder" when used in this First Amendment shall refer to
        this First Amendment as a whole and not to any particular Article,
        Section, subsection or provision of this First Amendment.

              (b)  Section, subsection and Exhibit references herein are to such
        Sections, subsections and Exhibits to this First Amendment unless
        otherwise specified.






<PAGE>   3
                   ARTICLE II.  AMENDMENTS TO CREDIT AGREEMENT

        Borrower, Agent and Lenders agree that the Credit Agreement is hereby
amended, effective as of the First Amendment Effective Date, in the following 
particulars.

        Section 2.01    Amendments and Supplements to Definitions.

              (a)  The term "Agreement," as defined in the Credit Agreement, is
        hereby amended to mean the Credit Agreement, as amended and supplemented
        by this First Amendment and as the same may from time to time be further
        amended or supplemented.

              (b)  Section 1.02 of the Credit Agreement is hereby further 
        amended and supplemented by adding the following new definitions where
        alphabetically appropriate, which read in their entirety as follows:

                   "First Amendment" shall mean that certain First Amendment
              and Supplement to Credit Agreement dated effective as of the
              First Amendment Effective Date, by and among Borrower, Agent and
              Lenders.

                   "First Amendment Effective Date" shall mean the date upon
              which the Borrower accepts a commitment from Scotiabank for the
              refinancing of the Indebtedness upon terms mutually acceptable to
              the Borrower and Scotiabank.

                   "Preferred Stock" shall mean the shares of non-convertible
              up to 13% cumulative preferred stock, par value $.01 per share,
              issued or to be issued by the Borrower during the period of one
              year from and after the First Amendment Effective Date.

        Section 2.02    Amendments and Supplements to Article VII. The first
full sentence of Section 7.07 of the Credit Agreement is hereby amended and
restated in its entirety to read as follows:

                   "Proceeds of the Loans may be used by Borrower for general
              corporate purposes including, but not limited to, the payment of
              reimbursement obligations under Section 2.09, and $35,000,000 in
              proceeds of Loans may be used by Borrower for a period of one
              year from and after the First Amendment Effective Date to
              purchase shares of its issued and outstanding common stock;
              provided, however, it is hereby understood and agreed that,
              except as provided in this Section 7.07, neither Agent nor
              Lenders shall have any obligation to make any future Loans to
              Borrower for the purpose of purchasing any of its Capital Stock."

        Section 2.03    Amendments and Supplements to Article IX. Section 9.04
of the Credit Agreement is hereby amended in its entirety to read as follows:


                                     -2-


<PAGE>   4


                 "Section 9.04    Restricted Payments. Neither the Borrower nor
         any Restricted Subsidiary will declare or pay any dividend, purchase,
         redeem or otherwise acquire for value any of its stock now or
         hereafter outstanding, return any capital to its stockholders or make
         any distribution of its assets to its stockholders or repay, purchase,
         repurchase, defease or make any prepayments on the Senior Unsecured
         Notes; provided, however, so long as no Default has occurred and is
         continuing hereunder or would occur as a consequence thereof:

                          (a)     for a period of one year from and after the
                 First Amendment Effective Date, the Borrower may purchase
                 shares of its issued and outstanding common stock, in an
                 aggregate amount not to exceed $85,000,000, $35,000,000 of the
                 purchase price of which may be paid using the proceeds of
                 Loans to the Borrower hereunder, with the balance to be paid
                 out of the proceeds received by the Borrower from the issuance
                 of the Preferred Stock; provided further that, except as set
                 forth in this Section 9.04(a), neither Agent nor Lenders shall
                 have any obligation to permit Borrower to purchase any further
                 shares of its Capital Stock; and

                          (b)     the Borrower and any Restricted Subsidiary
                 may declare and pay (i) dividends payable in Capital Stock of
                 the Borrower, or (ii) dividends or distributions payable to
                 the Borrower or any Restricted Subsidiary; provided further
                 that the Borrower may (x) declare and pay cash dividends on
                 the common stock of the Borrower in an amount not exceeding
                 (A) with respect to cash dividends declared during the 1996
                 fiscal year, $5,000,000; and (B) with respect to cash dividends
                 declared during each fiscal year after 1996, the lesser of (1)
                 $5,000,000 per annum or (2) an amount which, together with all
                 other cash dividends declared after December 31, 1996, shall
                 not exceed fifty (50%) percent of aggregate Consolidated Net
                 Income for the period (computed as a single period) from
                 January 1, 1996 through the last day of fiscal quarter
                 preceding the date on which such cash dividend was declared,
                 and (y) declare and pay quarterly cash dividends on
                 $50,000,000 of the Preferred Stock of Borrower at the
                 specified rate not in excess of 13% per annum."

                           ARTICLE III. MISCELLANEOUS

         Section 3.01.    Adoption, Ratification and Confirmation of Credit
Agreement. Each of the Borrower, Agent, and Lenders does hereby adopt, ratify
and confirm the Credit Agreement, as amended hereby, and acknowledges and
agrees that the Credit Agreement, as amended hereby, is and remains in full
force and effect.

         Section 3.02     Successors and Assigns. This First Amendment shall be
binding upon and inure to the benefit of the parties hereto and their
respective successors and assigns permitted pursuant to the Credit Agreement.

                                      -3-
<PAGE>   5
         Section 3.03     Counterparts. This First Amendment may be executed by
one or more of the parties hereto in any number of separate counterparts, and
all of such counterparts taken together shall be deemed to constitute one and
the same instrument and shall be enforceable as of the First Amendment
Effective Date upon the execution of one or more counterparts hereof by
Borrower, Agent and Lenders. In this regard, each of the parties hereto
acknowledges that a counterpart of this First Amendment containing a set of
counterpart execution pages reflecting the execution of each party hereto shall
be sufficient to reflect the execution of this First Amendment by each
necessary party hereto and shall constitute one instrument.

         Section 3.04     Number and Gender. Whenever the context requires,
reference herein made to the single number shall be understood to include the
plural; and likewise, the plural shall be understood to include the singular.
Words denoting sex shall be construed to include the masculine, feminine and
neuter, when such construction is appropriate; and specific enumeration shall
not exclude the general but shall be construed as the cumulative. Definitions
of terms defined in the singular or plural shall be equally applicable to the
plural or singular, as the case may be, unless otherwise indicated.

         Section 3.05     Entire Agreement. This First Amendment constitutes
the entire agreement among the parties hereto with respect to the subject
hereof. All prior understandings, statements and agreements, whether written or
oral, relating to the subject hereof are superseded by this First Amendment.

         Section 3.06     Invalidity. In the event that any one or more of the
provisions contained in this First Amendment shall for any reason be held
invalid, illegal or unenforceable in any respect, such invalidity, illegality
or unenforceability shall not affect any other provision of this First
Amendment.

         Section 3.07.    Titles of Articles, Sections and Subsections. All
titles or headings to Articles, Sections, subsections or other divisions of
this First Amendment or the exhibits hereto, if any, are only for the
convenience of the parties and shall not be construed to have any effect or
meaning with respect to the other content of such Articles, Sections,
subsections, other divisions or exhibits, such other content being controlling
as the agreement among the parties hereto.

         Section 3.08     Governing Law. This First Amendment shall be deemed
to be a contract made under and shall be governed by and construed in
accordance with the internal laws of the State of New York.

         THIS FIRST AMENDMENT, THE CREDIT AGREEMENT, AS SUPPLEMENTED AND
         AMENDED HEREBY, THE NOTES, AND THE OTHER SECURITY INSTRUMENTS
         REPRESENT THE FINAL AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE
         CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL
         AGREEMENTS OF THE PARTIES.

         THERE ARE NO UNWRITTEN AGREEMENTS BETWEEN THE PARTIES.


                                      -4-
<PAGE>   6
         IN WITNESS WHEREOF, the parties hereto have caused this First
Amendment to be duly executed and delivered by their proper and duly authorized
officers as of the Effective Date.

BORROWER:                          REXENE CORPORATION
                                   
                                   By:  /s/ GEFF PERERA
                                      ---------------------------------------
                                   Name:  Geff Perera
                                        -------------------------------------
                                   Title:  Executive Vice President & Chief
                                           Financial Officer
                                         ------------------------------------
                                   
AGENT AND LENDERS:                 THE BANK OF NOVA SCOTIA, individually and
                                   as Agent
                                   
                                   By:   The Bank of Nova Scotia, Atlanta
                                         Agency
                                   
                                         By:  /s/ 
                                            ---------------------------------
                                         Name:  
                                              -------------------------------
                                         Title:  
                                               ------------------------------
                                   
                                   MELLON BANK, N.A.
                                   
                                   By:  /s/ 
                                      ---------------------------------------
                                   Name:  
                                        -------------------------------------
                                   Title:  
                                         ------------------------------------
                                   
                                   ABN AMRO BANK N.V. HOUSTON AGENCY

                                   By:   ABN AMRO NORTH AMERICA, INC., 
                                         as Agent
                                   
                                   
                                         By:  /s/ 
                                            ---------------------------------
                                         Name:  
                                              -------------------------------
                                         Title:  
                                               ------------------------------
                                   
                                   
                                         By:  /s/ 
                                            ---------------------------------
                                         Name:  
                                              -------------------------------
                                         Title:  
                                               ------------------------------
                                   

                                     -5-


<PAGE>   7
                                      DEN NORSKE BANK ASA

                                      By:                   
                                         ---------------------------------------
                                      Name:             
                                           -------------------------------------
                                      Title:                 
                                            ------------------------------------


                                      By:                  
                                         ---------------------------------------
                                      Name:              
                                           -------------------------------------
                                      Title:                       
                                            ------------------------------------


                                      CREDIT LYONNAIS NEW YORK BRANCH

                                      By:                  
                                         ---------------------------------------
                                      Name:             
                                           -------------------------------------
                                      Title:                 
                                            ------------------------------------



                                      SOCIETE GENERALE, SOUTHWEST AGENCY

                                      By:                 
                                         ---------------------------------------
                                      Name:               
                                           -------------------------------------
                                      Title:                 
                                            ------------------------------------


                                      NATIONAL BANK OF CANADA

                                      By:                  
                                         ---------------------------------------
                                      Name:               
                                           -------------------------------------
                                      Title:                  
                                            ------------------------------------


                                      By:                    
                                         ---------------------------------------
                                      Name:             
                                           -------------------------------------
                                      Title:                 
                                            ------------------------------------

                                      THE YASUDA TRUST AND BANKING
                                      COMPANY, LIMITED

                                      By:                 
                                         ---------------------------------------
                                      Name:                
                                           -------------------------------------
                                      Title:                 
                                            ------------------------------------



                                     -6-