A method for removing gaseous pollutants from a polluted flue gas stream of an oil or coal-fired combustion furnace. A source of liquified combustible gas and a combustible gas-fired furnace are provided. The polluted flue gas stream is passed through a dust removal apparatus and removes flue dust from the flue gas stream so as to provide a de-dusted flue gas stream. Heat is recovered from the flue gas to generate power therefrom. The de-dusted flue gas is compressed to a pressure of at least 10 psi gauge using the generated power to at least in part compress the de-dusted flue gas. The compressed flue gas and the combustible gas are passed through at least one multi-sided heat exchanger, where the compressed flue gas is first passed through one side thereof and the liquified combustible gas in an at least partially vaporized state is passed through another side thereof. The compressed flue gas is cooled sufficiently by the combustible gas that water and at least one gaseous pollutant of NO.sub.2, SO.sub.2 and CO.sub.2 are condensed and separated from the compressed flue gas to provide reduced pollutant flue gas. Fully vaporized combustible gas is passed to the combustible gas-fired furnace and combusted to a further flue gas and the further flue gas is passed into the polluted flue gas stream.