A system comprising: a combustion turbine engine that operatively discharges an exhaust stream; a stack fluidly connected to a downstream section of the combustion turbine engine; wherein the stack comprises: an opening through which some of the exhaust stream discharges into atmosphere; a stack penetration integrated with the stack, wherein the stack penetration directs an exhaust stream portion away from the opening; a stack return integrated with the stack, wherein the stack return returns the exhaust stream portion to the stack; a heat exchanger fluidly connected downstream of the stack penetration, wherein the heat exchanger comprises an exhaust inlet port that receives the exhaust stream portion originating at the stack penetration, and an exhaust outlet port that discharges the exhaust stream portion to the stack; an exhaust flow circuit fluidly connected between the stack and the heat exchanger; wherein the exhaust flow circuit comprises: an exhaust supply path comprising: a first end integrated with a discharge of the stack penetration; a stop valve located downstream of the first end, and a second end connected to the exhaust inlet port of the heat exchanger; an exhaust return path comprising: a first end connected to the stack return, and a second end connected to the exhaust outlet port of the heat exchanger; a fuel gas flow circuit fluidly connected to the heat exchanger; the fuel gas flow circuit comprising: a fuel supply path comprising: a supply end integrated with a fuel gas supply system; and a discharge end connected to a fuel inlet port of the heat exchanger; and a fuel return path comprising: a first end connected to a fuel outlet port of the heat exchanger; and a second end connected to a fuel system; and wherein the heat exchanger operatively transfers heat from the exhaust stream portion, flowing through the exhaust flow circuit to fuel gas, flowing through the fuel gas flow circuit, the system further comprising an exhaust recirculation circuit that fluidly connects the exhaust supply path and the exhaust return path. a combustion turbine engine that operatively discharges an exhaust stream; a stack fluidly connected to a downstream section of the combustion turbine engine; wherein the stack comprises: an opening through which some of the exhaust stream discharges into atmosphere; a stack penetration integrated with the stack, wherein the stack penetration directs an exhaust stream portion away from the opening; a stack return integrated with the stack, wherein the stack return returns the exhaust stream portion to the stack; an opening through which some of the exhaust stream discharges into atmosphere; a stack penetration integrated with the stack, wherein the stack penetration directs an exhaust stream portion away from the opening; a stack return integrated with the stack, wherein the stack return returns the exhaust stream portion to the stack; a heat exchanger fluidly connected downstream of the stack penetration, wherein the heat exchanger comprises an exhaust inlet port that receives the exhaust stream portion originating at the stack penetration, and an exhaust outlet port that discharges the exhaust stream portion to the stack; an exhaust flow circuit fluidly connected between the stack and the heat exchanger; wherein the exhaust flow circuit comprises: an exhaust supply path comprising: a first end integrated with a discharge of the stack penetration; a stop valve located downstream of the first end, and a second end connected to the exhaust inlet port of the heat exchanger; an exhaust return path comprising: a first end connected to the stack return, and a second end connected to the exhaust outlet port of the heat exchanger; an exhaust supply path comprising: a first end integrated with a discharge of the stack penetration; a stop valve located downstream of the first end, and a second end connected to the exhaust inlet port of the heat exchanger; an exhaust return path comprising: a first end connected to the stack return, and a second end connected to the exhaust outlet port of the heat exchanger; a fuel gas flow circuit fluidly connected to the heat exchanger; the fuel gas flow circuit comprising: a fuel supply path comprising: a supply end integrated with a fuel gas supply system; and a discharge end connected to a fuel inlet port of the heat exchanger; and a fuel return path comprising: a first end connected to a fuel outlet port of the heat exchanger; and a second end connected to a fuel system; and a fuel supply path comprising: a supply end integrated with a fuel gas supply system; and a discharge end connected to a fuel inlet port of the heat exchanger; and a fuel return path comprising: a first end connected to a fuel outlet port of the heat exchanger; and a second end connected to a fuel system; and wherein the heat exchanger operatively transfers heat from the exhaust stream portion, flowing through the exhaust flow circuit to fuel gas, flowing through the fuel gas flow circuit, the system further comprising an exhaust recirculation circuit that fluidly connects the exhaust supply path and the exhaust return path.