<PAGE>

                                                                   EXHIBIT 10.20

                              WAIVER AND CONSENT
                                      to
              Second Amended and Restated Master Shelf Agreement
                         (Western Gas Resources, Inc.)

     This WAIVER AND CONSENT (this "Consent") is entered into as of November 22,
2000, by and among Western Gas Resources, Inc., a Delaware corporation (the
"Company"), and The Prudential Insurance Company of America and Pruco Life
Insurance Company (together, "Prudential").

                            PRELIMINARY STATEMENTS

     1.  The Company and Prudential entered into a Second Amended and Restated
Master Shelf Agreement dated as of December 19, 1991 (effective January 31,
1996), as amended by Letter Amendment No. 1 dated November 21, 1997, Letter
Amendment No. 2 dated March 31, 1999, Limited Waiver, Consent, Release and
Amendment No. 3 dated June 1, 1999 and Amendment No. 4 dated August 25, 2000 (as
amended, the "Agreement"). Capitalized terms not otherwise defined herein shall
have the meanings specified in the Agreement, as amended hereby.

     2.  The Company is the sole shareholder of Pinnacle Gas Treating, Inc., a
Texas corporation ("PGT").

     3.  The Company's board of directors has approved the sale of all of the
issued and outstanding common stock of PGT for approximately $38,000,000 (the
"PGT Stock Sale").

     4.  In order to secure the Obligations (as defined in the hereinafter
defined PGT Guaranty), including the obligations of the Company under the Notes
and the Agreement, the Company caused PGT to execute and deliver to Prudential a
Guaranty in favor of Prudential together with all subsequent holders of the
Obligations (the "PGT Guaranty").

     5.  In order to secure the Secured Obligations (as defined in the Pledge
Agreement), including the obligations of the Company under the Notes and the
Agreement, the Company executed and delivered the Pledge Agreement pursuant to
which the Company pledged and granted a security interest to Prudential in,
among other things, the common stock of PGT.

     6.  The Company and PGT have requested that Prudential (i) terminate the
PGT Guaranty and otherwise grant a general release of PGT under the PGT Guaranty
and any other obligations and liabilities arising under all documents and
agreements delivered pursuant to the PGT Guaranty or in connection therewith and
(ii) release Prudential's security interest in the shares of capital stock of
PGT under the Pledge Agreement (the "Release").

     7.  Prudential is willing to grant the Release, subject to the condition
that the lenders parties to the NCNB Agreement grant a similar release of the
guaranty provided by PGT to such lenders parties to the NCNB Agreement and NCNB
and the respective security interests of such lenders parties to the NCNB
Agreement and NCNB in shares of common stock of PGT.

     8.  Prudential is the holder of 100 % of the outstanding principal amount
of the Notes issued under the Agreement.

     In consideration of the foregoing and other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the
parties hereto agree as follows:

     SECTION 1.  Release and Waiver.
                 -------------------

     (a) Consent and Release of PGT.  Effective as of the PGT Effective Date,
         ---------------------------
     Prudential hereby (i) releases and discharges PGT and its successors and
     assigns from all obligations and liabilities under the PGT Guaranty and all
     documents and agreements delivered pursuant to the PGT Guaranty or in
     connection therewith and further hereby
<PAGE>

     terminates the PGT Guaranty and (ii) releases and discharges 1,000 shares
     of common stock of PGT from the liens and security interests granted by the
     Company pursuant to the Pledge Agreement automatically and without further
     action by any party.

     (b)  Waiver.  Prudential hereby waives any Default or Event of Default
          -------
     arising under either clause (xvii) or clause (xviii) of paragraph 7A of the
     Agreement to the extent that any such Default or Event of Default may be
     deemed to have arisen out of the releases provided for in this Section 1.

     (c)  Delivery of Release Documents.  On the PGT Effective Date, Prudential
          ------------------------------
     shall execute and deliver to the Company (i) a document evidencing the
     termination of the PGT Guaranty and the Release and (ii) an instruction to
     NCNB as bailee to return to the Company all stock certificates issued by
     PGT which NCNB possesses under the Pledge Agreement as bailee for
     Prudential. The Company hereby agrees to deliver to Prudential, within 30
     days after the PGT Stock Sale, an amendment to the Intercreditor Agreement
     duly executed by the parties thereto confirming the matters described in
     Section 3.

     SECTION 2.   Amendment to Pledge Agreement.  Exhibit B of the Pledge
                  ------------------------------
Agreement is hereby deleted and replaced with Exhibit B attached hereto.

     SECTION 3.   Conditions of Effectiveness. T his Consent shall become
                  ----------------------------
effective as of the date (the "PGT Effective Date") when

                  (a)      Prudential shall have received all of the following:

                  (i)  duly executed counterparts of this Consent;

                              (ii)  copies of a waiver and consent to the NCNB
                  Agreement in similar form and substance to this Consent
                  certified as true and correct copies by the Company;

                  (iii)  the consent attached hereto, duly executed by each
                  Guarantor except PGT;

                  (iv)   a certificate of a duly authorized officer of the
                  Company dated the date of this Consent certifying: (i) that
                  all of the representations and warranties set forth in Section
                  4 hereof are true and correct at and as of the time of such
                  effectiveness; and (ii) as to such other corporate matters as
                  Prudential shall deem necessary; and

                  (v)    payment of $1,400 to compensate Prudential for its
                  allocable overhead for in-house legal support.

                  (b)      PGT Effective Date. The Company shall advise
                           -------------------
          Prudential that the PGT Stock Sale has been consummated and Prudential
          shall have received evidence satisfactory to Prudential, in its sole
          and absolute discretion, that each other Person then a party to the
          Intercreditor Agreement has released its guaranty from PGT and lien
          and security interest in the common stock of PGT.

     SECTION 4.  Representations and Warranties of Company.   As an inducement
                 ------------------------------------------
to Prudential to enter into this Consent, the Company represents and warrants as
follows:

     (a)  Representations.  The representations and warranties contained in
          ----------------
     paragraph 8 of the Agreement are true and correct at and as of the time of
     the effectiveness hereof (except as such representations and warranties
     have been modified by the transactions contemplated herein).

     (b)  No Material Adverse Change.  Except as previously disclosed to
          ---------------------------
     Prudential in writing, there has been no material adverse change in the
     business, property or assets,
<PAGE>

     condition (financial or otherwise) or operations of the Company and its
     Subsidiaries taken as a whole since December 31, 1999.

     (c) No Event of Default or Default.  As of the date of this Amendment, no
         -------------------------------
     Event of Default or Default exists.


     SECTION 5.  Miscellaneous.
                 --------------

     (a) Except as specifically amended herein, the Agreement shall remain in
     full force and effect, and is hereby ratified and confirmed.

     b)  Other than as expressly set forth herein, the execution, delivery and
     effectiveness of this Consent shall not operate as a waiver of any right,
     power or remedy of Prudential, nor constitute a waiver of any provision of
     the Agreement, the Notes, the Guaranties, the Pledge Agreement or any other
     document, instrument or agreement executed and delivered in connection with
     the Agreement.

     (c) The Company confirms its agreement, pursuant to paragraph 11 B of the
     Agreement, to pay promptly all expenses of Prudential related to this
     Consent and all matters contemplated hereby.

     (d) GOVERNING LAW.  THIS CONSENT SHALL BE CONSTRUED AND ENFORCED IN
         --------------
     ACCORDANCE WITH, AND THE RIGHTS OF THE PARTIES SHALL BE GOVERNED BY, THE
     LAW OF THE STATE OF NEW YORK.

     (e) This Consent may be executed in counterparts (including those
     transmitted by facsimile), each of which shall be deemed an original and
     all of which taken together shall constitute one and the same document.
     Delivery of this Consent may be made by telecopy of a duly executed
     counterpart copy hereof.

     IN WITNESS WHEREOF, the parties hereto have caused their duly authorized
officers to execute this Consent as of the day and year first above written.

                                      WESTERN GAS RESOURCES, INC.

                                      By: _____________________________________
                                      Name:  William J. Krysiak,
                                             Vice President-Finance


THE PRUDENTIAL INSURANCE
COMPANY OF AMERICA

By: ________________________________
Name:
Title:  Vice President

PRUCO LIFE INSURANCE COMPANY

By: ________________________________
Name:
Title:  Vice President
<PAGE>

                                                                       EXHIBIT B

                                    ISSUERS
                                    -------

CORPORATIONS
Issuer                               CERTIFICATE NO      NO. OF SHARES     CLASS
--------------------------------------------------------------------------------

MIGC, Inc.                                 3             100,000 common

Western Gas Resources -Texas, Inc.         3                 990 common

Western Gas Resources -Texas, Inc.         4                  10 common

Mountain Gas Resources, Inc.               A-3         1,000,834 common

Western Power Services, Inc.               1               1,000 common

Lance Oil & Gas Company, Inc.              1               1,000 common



LIMITED LIABILITY COMPANIES

Issuer                        Membership Interest
-------------------------------------------------

Western Gas Wyoming, L.L.C.           100%
<PAGE>

                         CONSENT TO WAIVER AND CONSENT

     Each of the undersigned is a Guarantor ("Guarantor" and, collectively,
"Guarantors") under separate guaranties (each being a "Guaranty") in favor of
The Prudential Insurance Company of America and Pruco Life Insurance Company
(together, "Prudential") with respect to the obligations of Western Gas
Resources, Inc. (the "Company") under that certain Second Amended and Restated
Master Shelf Agreement dated as of December 19, 1991 (effective as of January
31, 1996) as amended by Letter Amendment No. 1 dated November 21, 1997, Letter
Amendment No. 2 dated March 31, 1999, Limited Waiver, Consent, Release and
Amendment No. 3 dated June 1, 1999 and Amendment No. 4 dated August 25, 2000 (as
amended, the 'Agreement"). Prudential and the Company are entering into that
certain Waiver and Consent, dated as of November 22, 2000 (the "Waiver and
Consent"). Each of the undersigned hereby consents to the Amendment and each
hereby confirms and agrees that its Guaranty is, and shall continue to be, in
full force and effect and is hereby confirmed and ratified in all respects
except that, upon the effectiveness of, and on and after the date of this
consent, all references in the Guaranty of the undersigned to the "Agreement,"
"thereunder," "thereof," or words of like import referring to the Agreement
shall mean the Agreement as amended by the Waiver and Consent, as the same may
be further amended or modified from time to time.

Dated as of November 22, 2000.

LANCE OIL & GAS COMPANY, INC.
MGTC, INC.
MIGC, INC.
MOUNTAIN GAS RESOURCES, INC.
WESTERN GAS RESOURCES-TEXAS, INC.
WESTERN GAS WYOMING, L.L.C.


By: _________________________________
Name:   William J. Krysiak, as Vice President-
        Finance of each of the above-named companies.