In a method of this invention, a first oxidant flow including oxygen and/or air, is injected into an exhaust gas flow from a primary burn zone in a furnace combustion chamber. The first oxidant flow is injected into the exhaust gas flow in a secondary burn zone downstream from the primary burn zone, but inside of the furnace chamber so that heat released from combustion of the first oxidant flow can be used for the purpose to which the furnace is applied. Also, the injection of the first oxidant flow into the secondary burn zone combusts fuel pollutants that might otherwise be released from the furnace into the atmosphere. The method can also include mixing a gaseous hydrocarbon fuel flow with the exhaust gas flow from the secondary burn zone to reduce the NO.sub.x, level in the exhaust gas. Also, the method can include mixing a second oxidant flow including air and/or oxygen, with the exhaust gas, preferably after reducing the NO.sub.x. If the furnace is a regenerative type, the air flow can be mixed with the exhaust gas flow in proximity to the entrance of a regenerator to heat the regenerator. The invention also includes a related system.