Methods and systems are provided for converting methane in a feed stream to acetylene. The method includes removing at least a portion of hydrides of arsenic, phosphorus, antimony, silicon, and boron from a hydrocarbon stream. The hydrocarbon stream is introduced into a supersonic reactor and pyrolyzed to convert at least a portion of the methane to acetylene. The reactor effluent stream may be treated to convert acetylene to another hydrocarbon process. The method according to certain aspects includes controlling the level of hydrides of arsenic, phosphorus, antimony, silicon, and boron in the hydrocarbon stream.