A computational fluid dynamics (CFD) modeling study has been carried out. The study involved gaseous fuel combustion with associated chemical reactions, radiative heat transfer,. and turbulence. The three different combustion environments that were adopted experimentally in a 100 kW drop-tube firing unit were examined. One air-fired and two oxy-fuel-fired cases [21 vol % O-2 for one combustion case (OF21) and 27 vol % O-2 for the other combustion case (OF27)] were investigated. A swirl injection system was used to achieve the flame stability of the turbulent non-premixed combustible gases. A modified eddy breakup (EBU) model was used with appropriate empirical coefficients for propane combustion reactions. The irreversible single-step and reversible multi-step reaction mechanisms were considered. The overall agreement of the CFD results with the available measured data was reasonable. The data compared were the temperature distributions and the species concentrations (CO2, CO, and O-2) at the most intensive combustion locations in the furnace. The luminous appearance and temperature levels of the OF27 flame were relatively close to the reference (air-fired) flame. This was due to a reduced volumetric flow rate and an increase in the O-2 concentration in the gas mixture. The carbon dioxide concentrations for both oxy-fuel-fired scenarios were around 8 times higher than that of the air-fired combustion case. The results obtained with the multi-step chemistry mechanism showed improved agreement, particularly in the flame zone. The concentration of CO was lower in the OF21 case. The unburnt fuel in the air-fired and OF27 cases was less than that of the OF21 case because of the low oxygen concentration used in the latter combustion case.. This study can provide a basis for the future investigation of combustion characteristics in a large-scale furnace under oxy-fuel-firing conditions.