Easy answer would be to determine experimentally. I would imagine the difficult answer involves looking at the part pressures of each individual component gas in the mixture to give you an air/fuel ratio for each component, and assuming that the component with the lowest LEL will ignite the rest (though reaction kinetics being complicated as they are, I doubt this would be correct!).
I don't know. Perhaps asking a mod nicely to move this to the chemistry house would be more useful?