A process and system for low NO.sub.x cogeneration to produce electricity and useful heat. Fuel and oxygen are provided to an internal combustion engine connected to drive an electric generator, to thereby generate electricity. An exhaust stream is recovered from the engine at a temperature of about 500.degree. to 1000.degree. F. which includes from about 6 to 15 percent oxygen. Fuel is added to the exhaust stream to create a fuel-rich mixture, the quantity of fuel being sufficient to react with oxygen and reduce the NO.sub.x in said exhaust stream. The fuel-enriched stream is provided to an afterburner, and the fuel, NO.sub.x and available oxygen are reacted to provide a heated oxygen-depleted stream. The oxygen-depleted stream is cooled in a first heat exchanger. Conversion oxygen is admixed with the cooled stream which is then passed over a catalyst bed under overall reducing conditions. NO is converted to NO.sub.2 at the forward end of the bed, and the NO.sub.2 then reacts on the remainder of the bed with excess combustibles. The stream from the reducing catalyst bed is cooled from a temperature of 750.degree. to 1250.degree. F. to about 450.degree. to 650.degree. F. and air is added to produce a further cooled stream at 400.degree. to 600.degree. F. having a stoichiometric excess of oxygen, and the further cooled stream is passed over an oxidizing catalyst bed to oxidize remaining combustibles. The resultant low NO.sub.x stream can then be provided for venting.