The present invention is directed to a method which significantly improves the efficiency of reducing nitrogen oxide formation and emission during incineration of a waste gas in an air-staged thermal oxidizer. In accordance with the present invention, a natural gas stream (12) is mixed with combustion air (18) in a burner (16) and the mixture is ignited with the immediate introduction of liquid water (19). Thus, the resulting mixture is then injected into a first reducing zone (2) which is fuel rich in order to begin the combustion process, but retard the formation of nitrogen oxides. The waste gas (8) exiting the reducing zone is deficient in oxygen due to the fuel rich atmosphere in the first reducing zone and cooler due to the water cooling as it enters the second oxidizing zone. In the second oxidizing zone (4), additional oxygen in the form of air (20) is injected to complete the combustion process. Due to the fact that the waste gas is cooler in the oxidizing zone, the peak temperature resulting from completion of combustion reactions is lower and thermal nitrogen oxide formation is minimized in the second oxidizing zone. In another embodiment, the method of the present invention further includes the step of mixing chemical reagents (25,26) with the cooling water prior to injection into either the reducing zone, the oxidizing zone, or both, to chemically reduce nitrogen oxides present in gases emanating from the reducing zone and to reduce formation of nitrogen oxides in the oxidizing zone.