Why would a new catalytic converter be heating up in a 1989 Buick Skylark with new plugs and muffler?

Catalytic converters run a couple hundred degrees hotter than anything else that has to do with the engine, or exhaust system. It is just the nature of their design that cause them to have a high temperature. Thats weird - catalytics usually warm up when they are active, meaning that they are working on cleaning the junk from the engine. So either you have a misadjusted - polluting engine - or the mufler is blocking some of the output and they are incresing the heat. Catalytic converters do tend to run hot by the way. On many cars there is also a feedback mechanism - via oxigen detectors which may be not working so the feedback to the engine is faulty and it is giving erroneous input to the engine management system.