Simulations on the effect of firing angle have been performed on a full-scale gas-fired industrial boiler. The investigation is done using a general-purpose CFD code, CFD ACE+ developed by CFD Research Corporation. The boiler is a tangential-fired furnace, where natural gas and air are injected at its four corners. The analysis emphasized on the combustion gas flow pattern and temperature distribution at furnace re-heater section, where steam temperature balance between left and right side is critical. The results showed that the flow of combustion gas inside the furnace is highly swirling with intense mixing of fuel and air. The combustion gas follows a helical pattern in anti-clockwise direction towards the furnace exit. Temperature distribution at furnace re-heater is highly influenced by the firing angle of air nozzle. Any misalignment in air firing angle will result in uneven temperature distribution between the left and right side of the re-heater.