This paper deals with a furnace which its fuel (gas oil) is replaced by natural gas in such a way that the flue gas temperature and the rate of heat removal from the furnace are remained unchanged. The question now is: what are the benefits due to replacement of the fuel of the furnace? The objective of this work is to answer this question. Modeling the furnace similar to a continuous flow stirred-tank reactor, the computational results show that the production of NO for equivalent natural gas is considerably less than that for gas oil. It is also obtained that the production rates of CO2 for both gas oil and natural gas are approximately close together. Considerable cost reduction is obtained by using natural gas in gas oil furnaces. (C) 2015 Elsevier Ltd. All rights reserved.