Exhibit
4.21




Supplemental
Indenture (this "Supplemental
Indenture"), dated as of September 24, 2009, among Targa Co-Generation
LLC (the "Guaranteeing
Subsidiary"), Targa Resources Partners LP, a Delaware limited partnership
("Targa Resources
Partners"), and Targa Resources Partners Finance Corporation ("Finance Corporation" and,
together with Targa Resources Partners, the "Issuers"), the other
Guarantors (as defined in the Indenture referred to herein) and U.S. Bank
National Association, as trustee under the Indenture referred to below (the
"Trustee").




WITNESSETH




WHEREAS,
the Issuers have heretofore executed and delivered to the Trustee an indenture
(the "Indenture"),
dated as of June 18, 2008 providing for the issuance of 81⁄4% Senior Notes due
2016 (the "Notes");




WHEREAS,
the Indenture provides that under certain circumstances the Guaranteeing
Subsidiary shall execute and deliver to the Trustee a supplemental indenture
pursuant to which the Guaranteeing Subsidiary shall unconditionally guarantee
all of the Issuers' Obligations under the Notes and the Indenture on the terms
and conditions set forth herein (the "Note Guarantee");
and




WHEREAS,
pursuant to Section 9.01 of the Indenture, the Trustee is authorized to execute
and deliver this Supplemental Indenture.




NOW,
THEREFORE, in consideration of the foregoing and for other good and valuable
consideration, the receipt of which is hereby acknowledged, the Guaranteeing
Subsidiary and the Trustee mutually covenant and agree for the equal and ratable
benefit of the Holders of the Notes as follows:




1.           Capitalized
Terms.  Capitalized terms used herein without definition shall
have the meanings assigned to them in the Indenture.




2.           Agreement to
Guarantee.  The Guaranteeing Subsidiary hereby agrees to
provide an unconditional Guarantee on the terms and subject to the conditions
set forth in the Note Guarantee and in the Indenture including but not limited
to Article 10 thereof.




3.           No Recourse Against
Others.  No past, present or future director, officer,
employee, incorporator, stockholder or agent of the Guaranteeing Subsidiary, as
such, shall have any liability for any obligations of the Issuers or any
Guaranteeing Subsidiary under the Notes, any Note Guarantees, the Indenture or
this Supplemental Indenture or for any claim based on, in respect of, or by
reason of, such obligations or their creation.  Each Holder of the
Notes by accepting a Note waives and releases all such liability.  The
waiver and release are part of the consideration for issuance of the
Notes.  Such waiver may not be effective to waive liabilities under
the federal securities laws and it is the view of the SEC that such a waiver is
against public policy.




4.           NEW YORK LAW TO
GOVERN.  THE LAW OF THE STATE OF NEW YORK SHALL GOVERN AND BE
USED TO CONSTRUE THIS SUPPLEMENTAL INDENTURE.




5.           Counterparts.  The
Parties may sign any number of copies of this Supplemental
Indenture.  Each signed copy shall be an original, but all of them
together represent the same agreement.




6.           Effect of
Headings.  The Section headings herein are for convenience only
and shall not affect the construction hereof.




7.           The
Trustee.  The Trustee shall not be responsible in any manner
whatsoever for or in respect of the validity or sufficiency of this Supplemental
Indenture or for or in respect of the recitals contained herein, all of which
recitals are made solely by the Guaranteeing Subsidiary and the
Issuers.




IN
WITNESS WHEREOF, the parties hereto have caused this Supplemental Indenture to
be duly executed and attested, all as of the date first above
written.




Dated:
September 24, 2009.









TARGA
      CO-GENERATION LLCBy:
            /s/
      Matthew J. MeloyName:
            Matthew
      J. MeloyTitle:
            Vice
      President – Finance and Treasurer








TARGA
      RESOURCES PARTNERS LP
              
              By:  Targa
      Resources GP LLC,
              Its
      General PartnerBy:
            /s/
      Matthew J. MeloyName:
            Matthew
      J. MeloyTitle:
            Vice
      President – Finance and Treasurer








TARGA
      RESOURCES PARTNERS FINANCE CORPORATIONBy:
            /s/
      Matthew J. MeloyName:
            Matthew
      J. MeloyTitle:
            Vice
      President – Finance and Treasurer








U.S. BANK NATIONAL
      ASSOCIATION,
              as
      TrusteeBy:
            /s/
      Steven FinkleaAuthorized
      Signatory