- Condensation in the presence of non-condensable gases plays an important role in the nuclear industry. The RELAP5/MOD3 thermal hydraulic code was used to study the ability of the code to predict this phenomenon. Four separate effects experiments were simulated using this code. These were the Massachusetts Institute of Technology's (MIT) Pressurizer Experiment, the MIT Steam Condensation Experiment, the MIT Single Tube Experiment and the University of California, Berkeley (UCB) Steam Condensation Experiment. A new iterative approach to calculate the interface temperature and the degraded heat transfer coefficient was developed and implemented in the RELAP5/MOD3 thermal hydraulic code. This model employs the heat and mass transfer analogy since it considers the sensible and condensation heat transfer simultaneously. This model was found to perform much better than the reduction factor approach. The calculations using the new model were found to be in better agreement with the experimental values.