Horizontal-tube falling film condensers are used in many fields: chemical industries, refrigeration, desalination, cooling, etc. However, their performances cannot be predicted precisely due to the complexity of the phenomena. In this paper, heat and mass transfer in a horizontal-tube failing-film condenser used in an innovative desalination plant was studied theoretically. The polypropylene exchanger was designed to work at relatively high temperatures (25-35degreesC). In fact, the desalination unit was designed to function in remote areas where the ambient temperature frequently exceeds 30degreesC [1]. The model developed uses basic aerodynamic, hydrodynamic and heat/mass transfer information to predict the performance of the exchanger. The predicted transfer characteristics obtained from the simulations were compared to experimental data [2]. From this comparison, it was seen that the model is well able to predict the trends of heat and mass characteristics of the condenser. The influence of the different thermal, hydrodynamic and geometric parameters on the condenser performances was investigated. The variations of the distilled water amount inside the exchanger were analysed. The results were used to determine the optimum operational conditions. Finally, the model was also used to optimise the different exchanger components.