A port assembly for controlling the delivery of gases into the horizontal rotating reactor such as kiln gasifier is disclosed for introducing reactant gases. The port assembly comprises a cylindrical conduit is divided into noncommunicating four or more sections extending through the entire length of the kiln and supported by the stationary end plates of the rotating kiln gasifier. Each section of the conduit communicates with external supply of the reactant gases and each supply of reactant gases is independently controlled in terms of the composition and quantity. Each section of the port assembly communicates with the interior of the kiln gasifier through the plurality of nozzles are confined in the lower part of the conduit. The number and the size of the nozzles in individual section of the conduit is based on the desired flow of gases and available pressure for the supply of the reactant gases.