The retrofitting of an existing methanol or methanol/ammonia plant to make acetic acid is disclosed. The existing plant has a reformer to which natural gas or another hydrocarbon and steam (water) are fed. Syngas is formed in the reformer. All or part of the syngas is processed to separate out carbon dioxide, carbon monoxide and hydrogen, and the separated carbon dioxide is fed either to the existing methanol synthesis loop for methanol synthesis, or back into the feed to the reformer to enhance carbon monoxide formation in the syngas. Any remaining syngas not fed to the carbon dioxide separator can be converted to methanol in the existing methanol synthesis loop along with carbon dioxide from the separator and/or imported carbon dioxide, and hydrogen from the separator. The separated carbon monoxide is then reacted with the methanol to produce acetic acid or an acetic acid precursor by a conventional process. Also disclosed is the reaction of separated hydrogen with nitrogen, in a conventional manner, to produce ammonia. Also disclosed is the reaction of a portion of the acetic acid in a conventional manner with oxygen and ethylene to form vinyl acetate monomer. The nitrogen for the added ammonia capacity in a retrofit of an original methanol plant comprising an ammonia synthesis loop, and the oxygen for the vinyl acetate monomer process, are obtained from a new air separation unit.