A combined-cycle power system comprising: a steam turbine comprising a steam flow control valve, said steam turbine coupled to at least one electric generator; a steam source comprising a thermal energy reservoir, said steam source in flow communication with said steam turbine through said steam flow control valve; a combustion turbine coupled to the at least one electric generator said combustion turbine comprising an inlet guide vane; and a controller communicatively coupled to said steam flow control valve, said steam source, and said inlet guide vane, wherein said controller is configured to: determine a current thermal energy capacity of the thermal energy reservoir; determine a rate of frequency recovery available using the current thermal energy capacity of the thermal energy reservoir and a predetermined rate of change of the steam turbine control valve; and determine whether the rate of frequency recovery available is less than a required rate of frequency recovery derived from a grid frequency deviation and a predetermined recovery period, wherein the controller is configured to open only steam turbine control valve when the required rate of frequency recovery is less than the rate of frequency recovery available from the thermal energy reservoir; the controller is configured to open the inlet guide vane substantially simultaneously with said steam flow control valve in response to a grid under-frequency condition in which a required rate of frequency recovery exceeds the rate of frequency recovery available of the thermal energy reservoir; and the controller is configured to close the inlet guide vane substantially simultaneously with said steam flow control valve in response to a grid over-frequency condition in which a required rate of frequency recovery exceeds the rate of frequency recovery available of the thermal energy reservoir. a steam turbine comprising a steam flow control valve, said steam turbine coupled to at least one electric generator; a steam source comprising a thermal energy reservoir, said steam source in flow communication with said steam turbine through said steam flow control valve; a combustion turbine coupled to the at least one electric generator said combustion turbine comprising an inlet guide vane; and a controller communicatively coupled to said steam flow control valve, said steam source, and said inlet guide vane, wherein said controller is configured to: determine a current thermal energy capacity of the thermal energy reservoir; determine a rate of frequency recovery available using the current thermal energy capacity of the thermal energy reservoir and a predetermined rate of change of the steam turbine control valve; and determine whether the rate of frequency recovery available is less than a required rate of frequency recovery derived from a grid frequency deviation and a predetermined recovery period, wherein determine a current thermal energy capacity of the thermal energy reservoir; determine a rate of frequency recovery available using the current thermal energy capacity of the thermal energy reservoir and a predetermined rate of change of the steam turbine control valve; and determine whether the rate of frequency recovery available is less than a required rate of frequency recovery derived from a grid frequency deviation and a predetermined recovery period, wherein the controller is configured to open only steam turbine control valve when the required rate of frequency recovery is less than the rate of frequency recovery available from the thermal energy reservoir; the controller is configured to open the inlet guide vane substantially simultaneously with said steam flow control valve in response to a grid under-frequency condition in which a required rate of frequency recovery exceeds the rate of frequency recovery available of the thermal energy reservoir; and the controller is configured to close the inlet guide vane substantially simultaneously with said steam flow control valve in response to a grid over-frequency condition in which a required rate of frequency recovery exceeds the rate of frequency recovery available of the thermal energy reservoir.