We have developed an analytical solution of reversible step growth polymerization in semibatch reactor with condensation product flashing. We have shown that the MWD of the polymer formed has Flory distribution, unaffected by flashing of condensation product, if the feed to the reactor is either pure monomer or has most probable MWD. We have determined analytical expression for the time when the evaporation of condensation product starts. Assuming the vapor liquid equilibrium (VLE) is given by Raoult's law (as well as the Flory–Huggins relation) we have determined the number and weight average chain lengths. We have subsequently analyzed the polycondensation step of the formation of polyethylene terephthalate and determined the effect of temperature and pressure on the course of polymerization.