Thanks for an awesome response. (I'll apologize now, as I'm on the go and responding from my phone--I'll try to make this as readable as possible, but there are bound to be mistakes.)
Definitely doesn't need to be on a global scale, it only needs to fill a few important roles. Essentially, the idea is to make exiting the Earth's atmosphere difficult, and entering again soon afterward almost impossible. The premise is that an asteroid is on a collision course with Earth, but the climatic conditions make it more worthwhile to evacuate than to develop a deflection mission.
Of course, the methane won't keep them out for long. It only takes about 8 years, from what I've read, for the methane too do its thing and go away, and of course it will be at less than flammable levels long before that. They only need to be stopped for the immediate future, since they are indeed given a reason to return.
The timeframe can be manipulated, so the only concern is plausibility.
I do need life to survive these conditions for the story to work. But, the people already spend most of their time indoors to avoid the harsh conditions, and I have toyed with the idea of augmenting their bodies with technology just to make the environment survivable (assist breathing, filtering out toxins, etc.)
The proposed source for this large influx of methane is lab grown meat. Increased global population with increased meat consumption, all in the face of disappearing farmland (to support more residential space), causes these labs to pop up everywhere.
So, can we have have pockets of methane, perhaps, that would make entering and exiting the atmosphere with any immediacy nearly impossible (or is there something else which could take the place of methane here)?
And then, could the presumed source plausibly create that much methane, and for how long (given current levels of oxygen, etc.)?
Lake Nyos is super intriguing. Don't think it's going to help me out here, but I'm definitely going to read up on the phenomenon.
Thanks for all the help you've been so far. You've given me a lot to think about.