Green chili is a catch-all designation that applies to several different varieties of green chili peppers, from the mild Anaheim to the smoky, medium-hot pasilla to the fiery serrano. Because of the variety to choose from, selecting a proper pepper substitution is simple. Taking the dish you are preparing into consideration, select a pepper whose flavor will complement the other ingredients, keeping in mind the spiciness so it isn't overwhelming.
If mild green chilies are more your speed than their fiery counterparts, go with Anaheim, banana or poblano peppers. Try using a pasilla, or a green fresno -- red fresnos are hotter -- if you'd like to add a noticeable kick of heat to your dish, but not overwhelm it with too much spice. A jalapeno's hotness can vary from medium-hot to hot, so taste test before incorporating it into your dish. And if you like it hot, heat things up with a serrano or Thai chili pepper.
If the dish you're making doesn't depend on the appearance or texture of green chili peppers, various spices can be substituted in a pinch. Try substituting chili powder, cayenne pepper or crushed red pepper for green chilies. Begin by adding just a few dashes of these potent spices, tasting and adding more spice until you are satisfied with the flavor and heat level.