A CFD study using STAR-CCM+ 7.04 is performed to simulate an industrial heater with burners firing refinery gas fuel. The heater has been reported with short run length due to coking inside the process tubes. Heater, burners and combustion air duct are included in the modeled geometry. Turbulence, combustion and radiation models are activated to simulate the environment inside the heater. System analysis is performed such as combustion flame pattern, temperature and velocity fields, heat flux and temperature distribution at various locations etc. Non-uniform heat flux distribution on process tubes is identified and possible solutions are suggested.