A three-dimensional model with the laminar flow of an incompressible viscous gas at a steady-state is developed to simulate a urea-SCR system by the SIMPLE algorithm. A porous medium coated by a metal-oxide-based catalyst is considered in this study. The flow field and chemical reactions inside the reactor are calculated simultaneously by a porous medium approach. In a urea-SCR modeling, the gas transport properties exist as parameters in each of the conservation equations. The evaluations of density, diffusion coefficients, viscosities, thermal conductivities and specific heats are required to select the most suitable gas transport properties in a numerical modeling of a multi-component gaseous mixture and chemically reacting flow. The purpose of this study is to find the gas transport property that exerts the strongest influence on the processes of NOx reduction and urea gas decomposition in a urea-SCR system by comparing the simulation results using the temperature-dependent gas transport properties with those of the constant ones. The effect of urea gas decomposition process is also investigated by comparing the simulation results using the urea gas with those of the ammonia one as a reducing agent. It is found that the gas transport property with the strongest influence on the processes of NOx reduction and urea gas decomposition in a urea-SCR system is the density. Therefore, the density with temperature-dependent function could not be neglected in the multi-component gaseous mixture and chemically reacting flow of a urea-SCR system. The diffusion coefficient has a weak influence on the processes of NOx reduction and urea gas decomposition. Nevertheless, the temperature-dependent diffusion coefficient should not be excluded from the modeling of a urea-SCR system especially at high working temperatures. The other temperature-dependent gas transport properties can be exchanged with the constant ones. It is also found that the inaccuracy in the SCR modeling using a constant density is higher as the process of urea gas decomposition is included.