2000 Chevy Monte Carlo intake manifold

If it is a 3.8L with the plastic intake, it would be from a coolant leak around the EGR passage in the manifold. The leak will cause the coolant to mix with oil and degrade the plastic when combined with the heat from the EGR passage. The exhaust gasses end up burning through tha manifold and catck the oil on the outside of the engine on fire.
There was a design change in the upper and lower manifolds to prevent this problem.
It is very important to get your leaks fixed before it gets to this point. This would have prevented a fire.