All MREs (meals, ready-to-eat) issued by the US government, and some civilian MREs, come with a flamess ration heater, or FRH. The FRH contains a compound that, when water is added, thoroughly heats the MRE in less than 20 minutes. If you don't have an FRH to heat your MRE, there are a few ways to heat them even without a microwave or a pot of boiling water.
Open your MRE package. Remove the entree pack to be heated.
Place packet under your clothing, against the skin of your torso. Keep in place for an hour or more. Body heat won't make the MRE hot, but it will keep it from being cold.
Put MRE near a heat source, if one is available, to make a heater that will make the MRE hot. Heat sources include an automobile engine, a rock that's been heated by a campfire or a radiator.
Do not put the MRE package over open flame.