In this study, exergy analysis and second law efficiency of gas heaters with reactive combustion processes are considered. The reactive combustion of natural gas is modeled via a chain of eight chemical reactions along with first and second law of thermodynamics. The main constitutional elements react in combustion chamber is considered at constant pressure and temperature of the basic elements hydrogen, carbon, oxygen, and nitrogen from the fuel and air. Heat transfer is modeled assuming free convection and radiation mechanisms from gas heater outer surfaces. The governed equations were solved using Engineering Equation Solver (EES) software tool. Results of this study reveal that preheating of air will considerably increase the second law efficiency of combustion process. Excessive air is effective up to 40 percent to prevent incomplete combustion; however, excessive air more than 80 percent is not recommended from exergy stand point of view in gas heaters.