Filmwise condensation plays an important role in many industrial applications. With the presence of noncondensable gas, the condensation process can be complicated and an appropriate modeling of the phenomena is necessary. This study takes the approach of modeling condensation with Computational Fluid Dynamics (CFD) based on species diffusion, with only the gas mixture modeled as the entire computational domain. The condensation rate is modeled as a sink term for the conservation equations. In addition, we suggest the use of free surface boundary condition, that is, a shear free boundary, as opposed to traditional approaches where the condensation is modeled to occur on a dry, no-slip wall. The free surface assumption provides better representation of the physical condensing interface without having to include both liquid and gas phases in the computational domain, which can bring great increase in computational cost with limited benefit. The free surface assumption is validated via case studies considering reasonable range of interfacial conditions, showing that the error from the free surface assumption can be quantified and is reasonably low for the considered range of conditions. Then, the developed condensation model and the free surface assumption for the condensing interface are implemented into the CFD codes and validated against a set of experimental data. The performance of the condensation model and the free surface assumption are assessed based on the prediction of the condensation heat transfer coefficient. The results show that the predictions are improved with applying the free surface boundary condition for the condensing interface compared to the results with the traditional no-slip boundary conditions. (C) 2018 Elsevier Ltd. All rights reserved.