Probably easier than you think, just add a few more ATM of CO2 and it will work out to be just like Venus.
Though -- it's actually a lot of CO2, for starters burning absolute every last bit of fossil fuel would probably contribute to ~0.1% of the amount needed for a runaway greenhouse event.
Though -- it's actually a lot of CO2, for starters burning absolute every last bit of fossil fuel would probably contribute to ~0.1% of the amount needed for a runaway greenhouse event.