<PAGE>   1
                                                                   EXHIBIT 10.70

                             SUBORDINATION AGREEMENT

         This Subordination Agreement (this "Agreement") is made as of February
7, 2001, by GMAC Commercial Credit LLC, a New York limited liability company
("Mortgage Holder") and TransTexas Gas Corporation, a Delaware corporation
(herein called "TransTexas") for the benefit of Mirant Americas Energy Capital,
LP, formerly named Southern Producer Services, L.P., and TCW DR VI Investment
Partnership, L.P. (herein collectively called the "Production Payment Owners").

                                    RECITALS:

         1. The Mortgage Holder and TransTexas have entered into that certain
Third Amended and Restated Accounts Receivable Management and Security Agreement
dated as of March 15, 2000, as amended by Amendment No. 1 to Third Amended and
Restated Accounts Receivable Management and Security Agreement dated as of
October 1, 2000 (herein, as from time to time amended or supplemented, called
the "ARM Agreement"). Pursuant to the ARM Agreement, TransTexas has executed and
delivered certain mortgages, deeds of trust, security agreements and financing
statements (collectively, the "Security Documents") to secure the Obligations
(as defined in the ARM Agreement and used herein with the same meaning). A true
and complete list of such Security Documents which burden properties in Texas is
attached as Schedule 1 hereto and made a part hereof. Each deed of trust and
mortgage listed on Schedule 1 is herein called a "Mortgage".

         2. As described in Section 9.2 of each Mortgage, TransTexas has
heretofore executed and delivered to the Production Payment Owners that certain
Production Payment Conveyance made effective as of 9:00 a.m., Houston, Texas
time on March 1, 2000 (defined in each such Section 9.2 as the "Production
Payment Conveyance" and herein called the "Original Conveyance"), pursuant to
which TransTexas conveyed and assigned to the Production Payment Owners a
certain production payment (defined in each such Section 9.2 as the "Production
Payment" and herein called the "Original Production Payment"). As described in
Section 9.2 of each Mortgage, the Original Conveyance was given pursuant to an
Order of the United States Bankruptcy Court. As contemplated in such Court
Order, the Original Conveyance has heretofore been supplemented by a First
Supplement to Production Payment Conveyance dated as of June 7, 2000 (the "First
Supplement"), by a Second Supplement to Production Payment Conveyance dated as
of September 8, 2000 (the "Second Supplement"), and by a Third Supplement to
Production Payment Conveyance dated as of November 7, 2000 (the "Third
Supplement"), to increase the unliquidated balance of the "Primary Sum"
thereunder. The Original Conveyance, the First Supplement, the Second
Supplement, and the Third Supplement have been recorded as set forth in Schedule
1 to Exhibit A hereto.

         3. Concurrently herewith, in consideration of funds to be paid to
TransTexas, TransTexas and the Production Payment Owners are executing and
delivering that certain Fourth Supplement to Production Payment Conveyance of
even date herewith (the "Fourth Supplement") which further supplements and
amends the Original Conveyance. Prior to the


<PAGE>   2

effective time of the Fourth Supplement, one of the three original owners of the
Production Payment (TCW Portfolio No. 1555 DR V Sub-Custody Partnership, L.P.)
is conveying all of its interests in the Production Payment to Mirant Americas
Energy Capital, LP, formerly named Southern Producer Services, L.P., one of the
two remaining Production Payment Owners, and after the effective time of the
Fourth Supplement, Mirant Americas Energy Capital, LP, formerly named Southern
Producer Services, L.P., is conveying part of its interests in the Production
Payment to TCW DR VI Investment Partnership, L.P., the other remaining
Production Payment Owner. True and correct copies of both such conveyances, and
a true and correct copy of the Fourth Supplement, are attached hereto as Exhibit
A-1, Exhibit A-2 and Exhibit A-3, respectively, and each is hereby made a part
hereof. As used herein, "Conveyance" means the Original Conveyance as amended
and supplemented by the First Supplement, the Second Supplement, the Third
Supplement, the Fourth Supplement, and such conveyances, and "Production
Payment" has the meaning given to such term in the Conveyance.

         4. Under the ARM Agreement, the Production Payment constitutes "First
Lien Debt" secured by a "Permitted Lien" (as such terms are defined in the ARM
Agreement). Pursuant to Sections 6(e) and 6(f) of the ARM Agreement, TransTexas
has delivered a "Subordination Request" to the Mortgage Holder, asking the
Mortgage Holder to enter into this Agreement in order to subordinate the
Mortgage Holder Liens to the Production Payment and the Conveyance (after taking
the Fourth Supplement into account), and TransTexas has satisfied the conditions
in such Sections 6(e) and 6(f). (As used herein, "Mortgage Holder Liens" means
all liens, security interests, assignments (of production or otherwise) and
other rights or interests of the Mortgage Holder at any time existing on or in
any properties or assets of TransTexas which are in any way subject to the
Production Payment or the Conveyance, whether such liens, security interests,
assignments and other rights or interests arise under the ARM Agreement or the
Security Documents or by operation of law or otherwise.)

         NOW, THEREFORE, in consideration of the foregoing, and in order to
induce the Production Payment Owners to enter into the Fourth Supplement and pay
funds to TransTexas in connection therewith, the Mortgage Holder and TransTexas
hereby agree as follows:

         Section 1. Subordination. The Production Payment and the proceeds
thereof are and hereafter shall be senior in right and priority to all Mortgage
Holder Liens, regardless of the relative priority of the Mortgage Holder Liens
and the Production Payment as determined without regard to this Agreement, and
the same shall have the same validity and effect as if the Conveyance (including
all supplements thereto) had been executed, delivered and properly recorded
prior to the date and time of execution of the ARM Agreement and the date and
time of execution and recordation of the Security Documents. All references in
Section 9.2 of each Mortgage to the "Production Payment Conveyance" shall
hereafter refer to the Conveyance, as defined herein, and all references in
Section 9.2 of each Mortgage to the "Production Payment Conveyance" shall
henceforth refer to the Production Payment Conveyance, as defined herein. As so
modified, the terms of Section 9.2 of each Mortgage are hereby ratified and
confirmed.

         Section 2. Acknowledgment of Reliance. The parties acknowledge and
agree that Production Payment Owners have entered into the Fourth Supplement in
reliance on the priority

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of the Conveyance, and thus are relying on the waiver, relinquishment and
subordination set forth herein.

         Section 3. Governing Law. This Agreement shall be deemed a contract and
instrument made under the laws of the State of Texas and shall be construed and
enforced in accordance with and governed by the laws of such state and the laws
of the United States of America, without regard to principles of conflicts of
law.

         Section 4. Invalidity of Particular Provisions. If any term or
provision of this Agreement shall be determined to be illegal or unenforceable,
all other terms and provisions hereof shall nevertheless remain effective and
shall be enforced to the fullest extent permitted by applicable law.

         Section 5. Successors and Assigns. This Agreement shall pass to and be
fully binding upon and inure to the benefit of the successors and assigns of
each party hereto (including the Production Payment Owners).

         Section 6. Counterparts. This Agreement may be separately executed in
any number of counterparts and by different parties hereto in separate
counterparts, each of which when so executed shall be deemed to constitute one
and the same instrument.

               IN WITNESS WHEREOF, this Agreement is executed by the parties
hereto on the dates set out below in their respective acknowledgments, to take
effect as of the date first above written.


MORTGAGE HOLDER:                       GMAC COMMERCIAL CREDIT LLC

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


TRANSTEXAS:                            TRANSTEXAS GAS CORPORATION

                                       By:
                                          --------------------------------------
                                          Ed Donahue, Vice President



                                       3
<PAGE>   4


STATE OF NEW YORK           )
                            )
COUNTY OF NEW YORK          )

         The foregoing instrument was acknowledged before me on this ____ day of
February, 2001, by ___________________________________, as
_______________________________ of GMAC Commercial Credit LLC, a New York
limited liability company, on behalf of such limited liability company.


        [SEAL]
                                       -----------------------------------------
                                       Notary Public, State of New York

My commission expires:

----------------------


STATE OF TEXAS              )
                            )
COUNTY OF HARRIS            )

         The foregoing instrument was acknowledged before me on this ____ day of
February, 2001, by Ed Donahue as Vice President of TransTexas Gas Corporation, a
Delaware corporation, on behalf of such corporation.


        [SEAL]
                                       -----------------------------------------
                                       Notary Public, State of Texas

My commission expires:

----------------------

                                       4
<PAGE>   5


                                   SCHEDULE 1
                                       TO
                             SUBORDINATION AGREEMENT

                        SCHEDULE OF TEXAS RECORDING DATA


                                  COUNTERPARTS
                                     OF THE
               MORTGAGE, DEED OF TRUST, ASSIGNMENT OF PRODUCTION,
                   SECURITY AGREEMENT AND FINANCING STATEMENT,
                          DATED AS OF OCTOBER 26, 2000,
            FROM TRANSTEXAS GAS CORPORATION, AS MORTGAGOR AND DEBTOR,
                           TO JACK MACGOWAN, TRUSTEE,
                 FOR THE BENEFIT OF GMAC COMMERCIAL CREDIT LLC,
                         AS MORTGAGEE AND SECURED PARTY,
                  ARE RECORDED IN THE REAL PROPERTY RECORDS IN
                    THE COUNTIES IN THE STATE OF TEXAS AS SET
                          FORTH IN THE FOLLOWING TABLE:

<TABLE>
<CAPTION>
        COUNTY IN WHICH                                                                RECORDING DATA
        COUNTERPART IS                                       (REFERENCE IS ALSO MADE TO THE "NOTE" IMMEDIATELY FOLLOWING THIS TABLE)
           RECORDED                    DATE          -------------------------------------------------------------------------------
    (EACH IN THE STATE OF               OF                                                                   REFERENCE / FILE /
            TEXAS)                  RECORDING        VOL/BK        PAGE(S)         FILM CODE NUMBER     INSTRUMENT / DOCUMENT NUMBER
    ---------------------           ---------        ------      ----------        ----------------     ----------------------------

<S>                                 <C>              <C>           <C>             <C>                  <C>
            AUSTIN                  11/08/00                                                                       007039

           BRAZORIA                 11/08/00                                                                      00048081

            CALHOUN                 11/08/00         266             82                                            65990

           CHAMBERS                 11/08/00        00-476          535                                            7249-B

           COLORADO                 11/08/00         353            326                                            11480

           GALVESTON                11/08/00                                         015-07-08175              GAC2000057358

            HARRIS                  11/08/00                                         ###-##-####                  U723726

            HIDALGO                 11/08/00                                                                       919748

             KENT                   11/08/00          6              85                                             792

           LIVE OAK                 11/08/00         389            214                                           00149763

             STARR                  11/08/00         0877           372                                            212227

           VAL VERDE                11/08/00         758         514 - 563                                        0184052

             WEBB                   11/08/00         0981           180                                            711667
</TABLE>


<PAGE>   6

<TABLE>
<CAPTION>
        COUNTY IN WHICH                                                                RECORDING DATA
        COUNTERPART IS                                       (REFERENCE IS ALSO MADE TO THE "NOTE" IMMEDIATELY FOLLOWING THIS TABLE)
           RECORDED                    DATE          -------------------------------------------------------------------------------
    (EACH IN THE STATE OF               OF                                                                   REFERENCE / FILE /
            TEXAS)                  RECORDING        VOL/BK        PAGE(S)         FILM CODE NUMBER     INSTRUMENT / DOCUMENT NUMBER
    ---------------------           ---------        ------      ----------        ----------------     ----------------------------

<S>                                 <C>              <C>           <C>             <C>                  <C>
            WHARTON                 11/08/00         390             35                                            209698

            ZAPATA                  11/08/00         641         001 - 066                                         121125
</TABLE>

NOTE: A COUNTERPART OF THE DEED OF TRUST REFERENCED ABOVE, WITH A COMPLETE
EXHIBIT "A," IS RECORDED IN THE REAL PROPERTY RECORDS OF GALVESTON COUNTY,
TEXAS, AS SET FORTH IN THE FOREGOING TABLE. AS TO THE COUNTERPARTS OF THE DEED
OF TRUST REFERENCED ABOVE THAT ARE RECORDED IN THE REAL PROPERTY RECORDS OF
OTHER COUNTIES IN THE STATE OF TEXAS (AS INDICATED IN THE TABLE ABOVE), THE
EXHIBIT "A" ATTACHED TO EACH SUCH RECORDED COUNTERPART IDENTIFIES ONLY THOSE
REAL PROPERTIES LOCATED IN THE COUNTY IN THE STATE OF TEXAS IN WHICH SUCH
COUNTERPART IS RECORDED. FOR A COMPLETE LIST OF ALL REAL PROPERTIES IN ALL
COUNTIES IN THE STATE OF TEXAS THAT ARE SUBJECT TO THE DEED OF TRUST REFERENCED
ABOVE, REFER TO, AND REFERENCE IS HEREBY MADE FOR ALL PURPOSES TO, THE
COUNTERPART OF THE DEED OF TRUST REFERENCED ABOVE THAT IS RECORDED IN THE REAL
PROPERTY RECORDS OF GALVESTON COUNTY, TEXAS.

                          ADDITIONAL SECURITY DOCUMENTS

SECURITY AND PLEDGE AGREEMENT DATED AS OF MARCH 15, 2000, BY AND BETWEEN
TRANSTEXAS GAS CORPORATION, A DELAWARE CORPORATION, AS DEBTOR, AND GMAC
COMMERCIAL CREDIT LLC, AS AGENT FOR THE "LENDERS" DEFINED AND DESCRIBED THEREIN.

FINANCING STATEMENT OF FORM UCC-1 FROM TRANSTEXAS GAS CORPORATION, AS DEBTOR, TO
GMAC COMMERCIAL CREDIT LLC, AS AGENT, AS SECURED PARTY, FILED MARCH 22, 2000, IN
THE OFFICE OF THE SECRETARY OF STATE OF THE STATE OF TEXAS UNDER FILE NUMBER
0000457016.

FINANCING STATEMENT OF FORM UCC-1 FROM TRANSTEXAS GAS CORPORATION, AS DEBTOR, TO
GMAC COMMERCIAL CREDIT LLC, AS AGENT, AS SECURED PARTY, FILED NOVEMBER 8, 2000,
IN THE OFFICE OF THE SECRETARY OF STATE OF THE STATE OF TEXAS UNDER FILE NUMBER
0000620473.

FINANCING STATEMENT OF FORM UCC-1 FROM TRANSTEXAS GAS CORPORATION, AS DEBTOR, TO
GMAC COMMERCIAL CREDIT LLC, AS AGENT, AS SECURED PARTY, FILED NOVEMBER 8, 2000,
IN THE OFFICE OF THE SECRETARY OF STATE OF THE STATE OF DELAWARE UNDER FILE
NUMBER 0079169.


<PAGE>   7

                                   EXHIBIT A-1

                                       TO

                             SUBORDINATION AGREEMENT

                  Conveyance of Interest in Production Payment


<PAGE>   8


                                   EXHIBIT A-2

                                       TO

                             SUBORDINATION AGREEMENT

                    Partial Conveyance of Production Payment


<PAGE>   9


                                   EXHIBIT A-3

                                       TO

                             SUBORDINATION AGREEMENT

               Fourth Supplement to Production Payment Conveyance