Homefront: The Revolution

The gas vents are shown by a green gas/smoke plume intermittently 'escaping' from underground.

Contrary to the achievement description, I got this using a molotov cocktail

This achievement is actually quite fun and simple to get.

Whenever you see plumes of green vapour/smoke rising from the ground this is a gas leak. As gas is extremely flammable this simply requires you to add the flame.

The best area to gain this achievement is Holloway Red Zone - Gassed Car park Strike point. To take over the car park you must burn the gas leaks, there are around 4 or 5 within a short area and with this being a strike point enemies are all over the place and can be easily lured to the area.

If you miss this Strike Point do not fear. I personally gained the achievementt in the Lombard Red Zone. Just past the first set of train tracks on the very bottom of the map. I came across four enemies gathered around a leaking gas main. I threw a Molotov in and the achievement popped. If you look at the bottom of the map, you will see a perfect circle shaperight next to the train tracks, the gas leak was there.

Whenever you find a leaking gas main if there are no enemies in the vicinity just start firing off your gun. If still none come, then stand over the gas, load up the map and add a custom waypoint for your current location, once you find enemies you will be able to easily the gas again.

You can use Flamethrower, Molotov, Any bullet firing gun to earn this achievement. However to increase your chances I found the Molotov and flame Thrower to be most effective. The explosion is pretty big so as long as the enemy is close to the gas it should be fine, they dont need to be exactly on the leaking gas.

If you, like me, couldn't pop the achievement somehow, wait until you reached Lombard. There is plenty of spots where you can get it. Align the smoke in front of a enemy and shoot with your gun.