A method of heating a fluid in a fluid heater integrated with a gas turbine. Fuel is burned in a radiant section of the fluid heater to heat the fluid and to produce combustion gases for the heating a downstream convective section. The combustion is supported by a combined oxidant stream. An oxygen containing stream or the combined stream, which is made up in part by the oxygen containing stream is preheated through indirect heat transfer with a first gas turbine exhaust stream produced by the gas turbine. A second gas turbine exhaust stream, also produced by the gas turbine is combined with the oxygen containing stream to form the combined oxidant stream. As such, the available heat energy supplied to the fluid heater is supplied not only by the combustion but also the gas turbine. This allows for the fuel to be conserved or for increased fluid throughput.