We present the equations for condensation in cooled upward laminar flow in tubes and consider their solution for low vapour concentrations and variable vapour-gas thermodynamic properties. We treated the full problem, including coupling with the aerosol size distribution, by using the PSI-CELL (Particle Source in Cell) method. The particle trajectories start from the point where the particles are generated by homogeneous nucleation. Particle size distribution and vapour scavenging by particles are obtained in forced convection and mixed convection regions. Calculations were also conducted with respect to tube diameters.