A method and apparatus for improving the energy efficiency of an existing gas turbine combined cycle plant, which includes a gas turbine having a compressor that pressurizes combustion air which is combusted with fuel in a combustion chamber to form combustion gases. The compressor is followed by a turbine, a high temperature heat exchanger, a low temperature heat exchanger, and a secondary process steam turbine II. Water from the secondary process steam turbine II is condensed in a following condenser-heat exchanger, pressurized by a following pump, and then is vaporized.