Abstract: A process is provided by the invention which comprises: (a) providing a tubular reactor having an inlet and an outlet, a furnace for heating the reactor, and a cooler having an inlet in communication with the reactor outlet and also having an outlet; (b) introducing a substantially constant flow of feed gas comprising steam to the reactor inlet while the reactor is heated by the furnace to produce a predetermined and substantially constant reactor outlet temperature; and (c) controlling, during at least a portion of (b), the concentration of an antifoulant in the feed gas based on cooler outlet temperature.