In this article a numerical investigation is performed on a two-phase closed thermosyphon. Particular attention is given to the condenser section. The working fluid is a binary mixture that consists of two components (R11 + R113) causing a variable conductance behavior in the thermosyphon. The mass, energy, and species conservation equations in conjunction with the overall mass conservation and continuity of momentum at liquid-vapor interface constraints and the thermodynamic equilibrium condition are solved numerically by use of the integral method. The effects of diffusion at the interface of effective and noneffective (shut-off) lengths in condenser are taken into account. The results of the present model are compared with available experimental data and it is found that there is a good agreement with experimental data at low evaporator power levels.