The atmosphere of Mars, for instance, is rich in carbon dioxide. Ethylene is a versatile feedstock for making plastics, pharmaceuticals and other chemicals. Carbon captured from the Martian atmosphere would be a precious resource, not a waste product.
I think the best space manufacturing scenario is a factory that converts a carbonaceous and wet asteroid into to plastic films that are coated with metal or silicon to make L1 sunshades for geoengineering, solar sails, photovoltaic arrays, etc. That kind of asteroid has carbonate rocks, "coal" and maybe even some trapped hydrocarbons which is somehow going to get upcycled to ethylene and other bulk chemicals, even edible synthetic fats, certainly you are going to get CO2 as a byproduct of chemical reactions which has to be recycled back into chemical feed stocks.