A method for separation of CO2 from the combustion gas from a thermal power plant fired with fossil fuel, wherein the combustion gas from the thermal power plant is used as cooled, compressed and reheated by combustion of natural gas in a combustion chamber to form an exhaust gas, where the exhaust gas is cooled an brought in contact with an absorbent absorbing CO2 from the exhaust gas to form a low CO2 stream and an absorbent with absorbed CO2, and where the low CO2 stream is heated by means of heat exchanges against the hot exhaust gas leaving the combustion chamber before it is expanded in turbines, is described. A plant for performing the method and a combined plant is also described.