A method of chemical vapor infiltration and deposition includes disposing a porous substrate within a reaction chamber, establishing a sub-atmospheric pressure within the reaction chamber, introducing a hydrocarbon reaction gas into a reaction zone of the reaction chamber to densify the porous substrate, withdrawing unreacted hydrocarbon reaction gas from the reaction chamber, the unreacted hydrocarbon reaction gas comprising hydrocarbon molecules having six or more carbon atoms, removing at least a portion of the hydrocarbon molecules having six or more carbon molecules from the unreacted hydrocarbon reaction gas by causing the portion of the hydrocarbon molecules having six or more carbon atoms to condense, and recirculating at least a portion of the unreacted hydrocarbon reaction gas back into the reaction zone.