This paper describes preliminary results of CFD modelling of the combustion processes and flow pattern in a 120 MW natural gas fired full-scale industrial boiler of an electricity generating plant. The natural gas and air are fired into the furnace tangentially at its four corners at 3 different elevation levels. The modelling was done using a general-purpose CFD software, CFD-ACE+ developed by CFD Research Corporation. The simulations were done in three stages: Fuel and air mixing simulation, gas fuel and air combustion simulation and combustion with heat transfer to the water walls simulation. The results of the simulation indicate that the flow inside the boiler is highly swirling with intense mixing. The flow follows approximately helical pattern following anti-clockwise direction. The temperature distribution prior to entry to the reheater is non-uniform and varies between 1600 K at the centre of the recirculation to 2150K, midway between the centre and the wall. The vertical velocity distribution is also non-uniform with lower velocity at the vortex core. The temperature and vertical velocity distributions are slightly asymmetric between the left and right part of the reheater. The results also revealed the importance of including the effect of heat transfer to the water walls as the temperature and flow patterns are significantly different with and without heat transfer effect.