Is my gas furnace correctly ducted/vented?
It is probably the combustion air vent. This is where the furnace gets its air to burn with the gas. Plugging it will make the furnace burn oxygen inside the building. If the building is sealed tightly it could cause a problem by not allowing the flue gases to move freely. (the air has to get in to allow the flue gas to get out) There are heat exchangers available to prevent the “cold air on the floor” problem, but your landlord probably didn’t want to spend the extra money for it. In short, do not plug that vent under any circumstances. Someone could die. I would however call the landlord and complain, you never know, he just might fix it.
