The purpose of this study is to develop a numerical method for simulating condensate flow problems observed in industrial processes. The preconditioning method developed by our group is applied to the calculations of very-slow flows assuming heterogeneous condensation in a mixing T-junction channel in which dry and moist air streams flowing into the channel are mixed together. A classical condensation model simplified by our group is used for evaluating condensation with heterogeneous nucleation. As shown in the calculated results, condensation in a T-junction pipe was variously influenced by the mixing type, such as stratified, turned jet, and wall jet flows. A notable difference in temperature distributions was also found with and without condensation due to the release of latent heat when condensation occurs.