A cooling system for turbine engines for controlling cooling of rotor cooling air and heating of fuel gas is disclosed. The cooling system may be formed from a housing formed from a main plenum, a rotor air cooler positioned at least partially in the main plenum and a fuel gas heater positioned at least partially in the main plenum downstream from rotor air cooler. The cooling system may include a by-pass plenum for returning used cooling air back to a region upstream of the rotor air cooler. An exhaust fluid control device may control fluid flow out of the first exhaust opening at the second end of the main plenum, and a by-pass fluid control device may control fluid flow into the by-pass inlet opening of the by-pass plenum. The control devices and a fan may control circulation of cooling fluids through the main plenum.