One of the challenges of the combustion process refers to the production of carbon monoxide and its sedimentation on coils and furnace walls. This phenomenon decreases the heat transfer efficiency of the furnace. In this paper, an industrial furnace faced with incomplete combustion is investigated. The orifice diameter, the fuel per oxygen ratio, and the velocity of the fuel jet are the parameters investigated. The simulation results show that the decrease of the flame length increases the maximum temperature of the flame and decreases the carbon monoxide production rate. In other words, the decrease of the flame length leads to decrease of the carbon monoxide mole fraction; however, the nitrogen oxides production rate increases. In addition, it is observed that the orifice diameter has... the maximum effect on the combustion process efficiency. The lowest diameter (0.5 mm) is selected as the optimum diameter, while CH4/O2 = 0.25 is chosen as the optimum value of the fuel/O2 ratio. The effects of the other parameters on the combustion process are negligible.