A system, comprising: a gas turbine engine; an exhaust processing system a selective catalytic reduction system; and a control system (46, 48) configured to: receive a desired value for NO x levels in an exhaust gas exiting the gas turbine engine and entering the SCR system, a desired value for reductant slip in the SCR system and a desired value for NO x levels in the processed exhaust gas exiting the exhaust processing system; (a) control a flow of reductant into the SCR system to achieve the desired reductant slip value; (b) determine whether an adjustment of the desired NO x value in the exhaust gas entering the SCR system is required based on measured values of the reductant slip in the SCR system and measured values of the composition of the processed exhaust stream exiting the exhaust processing system, and if so, to calculate the adjustment; (c) determine an adjustment of at least one operating parameter of the gas turbine engine to achieve the new desired value of NO x in the exhaust gas entering the SCR system; (d) control operation of the gas turbine engine based on the adjusted at least one operating parameter to achieve the desired value of NO x in the exhaust gas entering the SCR system; and (e) sequentially perform steps (a) (b), (c) and (d) in a loop a gas turbine engine; an exhaust processing system a selective catalytic reduction system; and a control system (46, 48) configured to: receive a desired value for NO x levels in an exhaust gas exiting the gas turbine engine and entering the SCR system, a desired value for reductant slip in the SCR system and a desired value for NO x levels in the processed exhaust gas exiting the exhaust processing system; (a) control a flow of reductant into the SCR system to achieve the desired reductant slip value; (b) determine whether an adjustment of the desired NO x value in the exhaust gas entering the SCR system is required based on measured values of the reductant slip in the SCR system and measured values of the composition of the processed exhaust stream exiting the exhaust processing system, and if so, to calculate the adjustment; (c) determine an adjustment of at least one operating parameter of the gas turbine engine to achieve the new desired value of NO x in the exhaust gas entering the SCR system; (d) control operation of the gas turbine engine based on the adjusted at least one operating parameter to achieve the desired value of NO x in the exhaust gas entering the SCR system; and (e) sequentially perform steps (a) (b), (c) and (d) in a loop receive a desired value for NO x levels in an exhaust gas exiting the gas turbine engine and entering the SCR system, a desired value for reductant slip in the SCR system and a desired value for NO x levels in the processed exhaust gas exiting the exhaust processing system; (a) control a flow of reductant into the SCR system to achieve the desired reductant slip value; (b) determine whether an adjustment of the desired NO x value in the exhaust gas entering the SCR system is required based on measured values of the reductant slip in the SCR system and measured values of the composition of the processed exhaust stream exiting the exhaust processing system, and if so, to calculate the adjustment; (c) determine an adjustment of at least one operating parameter of the gas turbine engine to achieve the new desired value of NO x in the exhaust gas entering the SCR system; (d) control operation of the gas turbine engine based on the adjusted at least one operating parameter to achieve the desired value of NO x in the exhaust gas entering the SCR system; and (e) sequentially perform steps (a) (b), (c) and (d) in a loop (a) control a flow of reductant into the SCR system to achieve the desired reductant slip value; (b) determine whether an adjustment of the desired NO x value in the exhaust gas entering the SCR system is required based on measured values of the reductant slip in the SCR system and measured values of the composition of the processed exhaust stream exiting the exhaust processing system, and if so, to calculate the adjustment; (c) determine an adjustment of at least one operating parameter of the gas turbine engine to achieve the new desired value of NO x in the exhaust gas entering the SCR system; (d) control operation of the gas turbine engine based on the adjusted at least one operating parameter to achieve the desired value of NO x in the exhaust gas entering the SCR system; and (e) sequentially perform steps (a) (b), (c) and (d) in a loop