A numerical description is developed of the process of film drainage and rupture during drop coalescence in liquid-liquid systems, The model is based on various simplifying approximations, corresponding to the regime in which interfacial mobility is controlled by the drop viscosity (partially mobile films), The constant force approach of a drop relative to another drop or to a free interface, taking into account the van der Waals attractive forces, is considered in this paper, The results are presented and compared with a simple analytical model proposed earlier for effective critical film-rupture thickness, which is found to provide a good first approximation.