Tangentially fired furnaces are vortex-combustion burners preferably used for oxy-fuel combustion in order to minimize the CO2 emission and control the boiler temperature. The objective of the present study is to investigate numerically the combustion and emission characteristics of oxy-fuel combustion of turbulent reacting flows in a three-dimensional model furnace of a 600 MW tangentially fired boiler. Numerical calculations of the flow field and thermal fields as well as the species concentrations for the oxy-fuel combustion in the furnace of a gaseous fuel tangentially fired boiler were conducted. Three cases of oxy-fuel combustion were investigated. These were 73%, 79%, and 65% of volumetric inlet CO2 while the remaining inlet volume was O-2. The results of these three oxy-fuel cases are compared to the air-fired combustion. In the present model, the radiation heat transfer was solved by the weighted sum gray gases approach, whereas the chemical reaction model was facilitated by a simple lumped reaction model. The results show that the oxy-fuel case of 79% CO2 can preferably minimize the combustion temperature for highly sustainable materials. However, the 65% CO2 oxy-fuel case generates high temperature values for improving the heat characteristics. The 73% CO2 oxy-fuel and the air-fired cases are fairly close to each other in thermal behavior.