This paper describes the method used to predict the combustion gas flow behaviour and temperature distribution in a full-scale boiler. A case study is presented where the analysis and prediction of temperature distribution inside the furnace is made. The boiler is a 120MW, natural gas-fired boiler. It employs a tangential firing system where all burners and air nozzles are directed to an imaginary circle at the furnace centre. The prediction is made based on the descretization of Navier-Stokes’ equations on the chosen computational domains. Generalpurpose CFD code, CFD ACE+ is used for the simulation. The prediction of combustion gas flow behaviour and temperature distribution inside the furnace shows reasonably good agreement with practical observation. Combustion gas inside the furnace is highly swirling in anti-clockwise direction. High level of mixing between natural gas and air is predicted. An imaginary fireball can be clearly observed located at the centre of the furnace. This fireball effect weakens as the furnace height increases. Temperature distribution inside the furnace shows non-uniform distribution where high temperature is shown in the region surrounding the imaginary fireball. Overall result shows that a CFD technique is capable to analyze the phenomena inside a boiler with reasonably high accuracy.