Ultra cleaning of combustion gas to near zero concentration of residual contaminants followed by the capture of CO.sub.2 is provided. The high removal efficiency of residual contaminants is accomplished by direct contact cooling and scrubbing of the gas with cold water. The temperature of the combustion gas is reduced to 0-20 degrees Celsius to achieve maximum condensation and gas cleaning effect. The CO.sub.2 is captured from the cooled and clean flue gas in a CO.sub.2 absorber (134) utilizing an ammoniated solution or slurry in the NH.sub.3--CO.sub.2H.sub.2O system. The absorber operates at 0-20 degrees Celsius. Regeneration is accomplished by elevating the pressure and temperature of the CO.sub.2-rich solution from the absorber. The CO.sub.2 vapor pressure is high and a pressurized CO.sub.2 stream, with low concentration of NH.sub.3 and water vapor is generated. The high pressure CO.sub.2 stream is cooled and washed to recover the ammonia and moisture from the gas.