A method for process monitoring and control of a chemical reactor in which a chemical reaction utilizing a halogenated selectivity modifier is performed includes: measuring a level of halogenated components in an inlet stream of a reactor inlet; measuring a level of halogenated components in an outlet stream of a reactor outlet; based on the level of halogenated components at the inlet stream and the outlet stream, determining a process performance indicator associated with a halogenated component; and adjusting an amount of halogenated selectivity modifier added to the reactor based on the process performance indicator.