A turbine nozzle cooling sub-system, said sub-system comprising: at least one turbine nozzle segment comprising an arcuate, radially outermost endwall, an arcuate, radially innermost endwall, and at least one airfoil vane, wherein said outermost endwall comprises at least one open passage, said innermost endwall comprises at least one open passage, said at least one airfoil vane extends between and is coupled to said inner radial endwall and said outer radial endwall, said vane further comprises a cavity, a leading edge, a trailing edge, and an airfoil vane external surface, said cavity comprises an airfoil vane internal surface and a plurality of turbulators, said cavity and said open passages being in flow communication to allow the flow of an airfoil cooling air stream; at least one diffuser in flow communication with a compressor assembly and said at least one turbine nozzle segment, said diffuser comprises at least one diffuser wall and at least one cavity, said at least one diffuser wall extending from the compressor assembly to said turbine nozzle segment to channel the airfoil cooling air stream to said at least one turbine nozzle segment, said airfoil cooling air stream comprising at least a portion of a compressor assembly discharge air stream flow; said turbine nozzle segment being in flow communication with a compressor discharge plenum to channel at least a portion of nozzle cooling air to said compressor discharge plenum, said compressor discharge plenum being in flow communication with a combustor assembly to channel at least a portion of the nozzle cooling air to said combustor assembly characterized in having an airfoil plenum wherein said airfoil plenum comprises a flow divider and a turbine nozzle cooling air compressor, said flow divider and turbine nozzle cooling air compressor in flow communication with the compressor assembly, wherein said flow divider facilitates channeling at least a portion of a compressor assembly discharge air stream flow to said turbine nozzle cooling air compressor, said compressor comprises at least one stage of compression and is in flow communication with said turbine nozzle segment. at least one turbine nozzle segment comprising an arcuate, radially outermost endwall, an arcuate, radially innermost endwall, and at least one airfoil vane, wherein said outermost endwall comprises at least one open passage, said innermost endwall comprises at least one open passage, said at least one airfoil vane extends between and is coupled to said inner radial endwall and said outer radial endwall, said vane further comprises a cavity, a leading edge, a trailing edge, and an airfoil vane external surface, said cavity comprises an airfoil vane internal surface and a plurality of turbulators, said cavity and said open passages being in flow communication to allow the flow of an airfoil cooling air stream; at least one diffuser in flow communication with a compressor assembly and said at least one turbine nozzle segment, said diffuser comprises at least one diffuser wall and at least one cavity, said at least one diffuser wall extending from the compressor assembly to said turbine nozzle segment to channel the airfoil cooling air stream to said at least one turbine nozzle segment, said airfoil cooling air stream comprising at least a portion of a compressor assembly discharge air stream flow; said turbine nozzle segment being in flow communication with a compressor discharge plenum to channel at least a portion of nozzle cooling air to said compressor discharge plenum, said compressor discharge plenum being in flow communication with a combustor assembly to channel at least a portion of the nozzle cooling air to said combustor assembly characterized in having an airfoil plenum wherein said airfoil plenum comprises a flow divider and a turbine nozzle cooling air compressor, said flow divider and turbine nozzle cooling air compressor in flow communication with the compressor assembly, wherein said flow divider facilitates channeling at least a portion of a compressor assembly discharge air stream flow to said turbine nozzle cooling air compressor, said compressor comprises at least one stage of compression and is in flow communication with said turbine nozzle segment.