The oxy-fuel-carbon dioxide combustion process is expected to drastically increase the energy efficiency and enable easy carbon sequestration. In this technology, the combustion products (carbon dioxide and water) are used to control the temperature and nitrogen is excluded from the combustion chamber, so that nitrogen oxide pollutants do not form. Therefore, in oxy-combustion, carbon dioxide and water are present in large concentrations in their transcritical state and may play an important role in kinetics. The computational chemistry methods may assist in understanding these effects, and molecular dynamics with a reactive force field (ReaxFF) seems to be a suitable tool for such a study. Here, we investigate applicability of the ReaxFF to describe the critical phenomena in carbon dioxide and water and find that several non-bonding parameters need adjustment. We report the new parameter set, capable of reproducing the critical temperatures and pressures. The critical isotherms of CO2/H2O binary mixtures are computationally studied here for the first time, and their critical parameters are reported.