A computational fluid dynamics (CFD) model of a 200 MW multifuel tangentially fired boiler has been developed using Fluent 6.3.26, which is able to model the three-fuel combustion system of coal, blast furnace gas (BFG), and coke oven gas (COG) with an eddy-dissipation model for simulating the gas-phase combustion. A level of confidence in the current CFD model has been established by carrying out a mesh independence test and validation against the experimental data obtained from the boiler for case study. The validated CFD model is then applied to investigate the effects of different BFG and COG flow rates on the boiler performance. It is found that increasing the BFG flow rate brings negative effects on the ignition of primary air and pulverized-coal mixture, pulverized-coal burnout, and heat transfer in the furnace and, consequently, decreases the thermal efficiency. However, increasing the COG flow rate can increase the thermal efficiency via improving the pulverized-coal burnout and heat transfer. Increasing both the BFG and COG flow rates is favorable for reducing NO emissions. The results also indicate that co-firing pulverized coal with BFG of about 20% heat input and COG of about 10% heat input is an optimal operating condition for improving the boiler performance at 180 MW load. The present study provides helpful information for understanding and optimizing the multifuel combustion of the boiler.