AlGaN metalorganic vapor-phase epitaxial growth simulation in a TMAl/TMGa/NH3/H2 system was studied to explain its growth rate and Al content. We proposed three kinds of Al- and Ga-containing polymers in this system. By using those polymers, we got good agreements in pressure, TMAl/(TMAl+TMGa) inlet ratio, and temperature dependences of AlGaN growth rate and its Al content between simulations and experiments, even at pressures higher than 40 kPa. Our results showed that the formation of these polymers is enhanced under higher pressures. The simulation considering those polymers could explain the linearity in the TMAl/(TMAl+TMGa) inlet ratio dependences of growth rate and Al content at lower pressure and their non-linearity at higher pressure.