A simple-cycle gas turbine system comprising an exhaust processing system configured to process exhaust gas generated by a gas turbine engine, wherein the exhaust processing system comprises a selective catalytic reduction system configured to reduce a level of nitrogen oxides (NO x) within the exhaust gas, the simple-cycle gas turbine system comprising: an injection system comprising a plurality of injection tubes configured to inject a fluid comprising cooling air into a transition duct of the exhaust processing system, and characterized in that the simple-cycle gas turbine system further comprises: a mixing system positioned adjacent to the plurality of injection tubes and within the exhaust processing system, wherein the mixing system comprises a mixing module having a plurality of turbulators configured to swirl the fluidand the exhaust gas in a first swirl direction to encourage turbulent flow along a longitudinal axis of the exhaust processing system and thereby facilitate mixing and heat exchange between the fluid and the exhaust gas, wherein the mixing module has a centerline axis crosswise to the longitudinal axis and a dimension perpendicular to the centerline axis and the longitudinal axis; wherein the mixing system is disposed within the transition duct and wherein the transition duct is fluidly coupled to a turbine of the gas turbine engine and is positioned upstream of an exhaust duct containing a catalyst of the SCR system, wherein each of the plurality of turbulators comprises a plurality of fins radially extending from a center of the turbulator, wherein a fin of the plurality of fins is configured to rotate about the longitudinal axis of the exhaust processing system in the first swirl direction, and wherein the plurality of turbulators are disposed along the dimension of the mixing module at various vertical and horizontal positions relative to the centerline axis of the mixing module. a mixing system positioned adjacent to the plurality of injection tubes and within the exhaust processing system, wherein the mixing system comprises a mixing module having a plurality of turbulators configured to swirl the fluidand the exhaust gas in a first swirl direction to encourage turbulent flow along a longitudinal axis of the exhaust processing system and thereby facilitate mixing and heat exchange between the fluid and the exhaust gas, wherein the mixing module has a centerline axis crosswise to the longitudinal axis and a dimension perpendicular to the centerline axis and the longitudinal axis; wherein the mixing system is disposed within the transition duct and wherein the transition duct is fluidly coupled to a turbine of the gas turbine engine and is positioned upstream of an exhaust duct containing a catalyst of the SCR system, wherein each of the plurality of turbulators comprises a plurality of fins radially extending from a center of the turbulator, wherein a fin of the plurality of fins is configured to rotate about the longitudinal axis of the exhaust processing system in the first swirl direction, and wherein the plurality of turbulators are disposed along the dimension of the mixing module at various vertical and horizontal positions relative to the centerline axis of the mixing module.