When a player purchases a power plant above the maximum limit, and must discard one, does the discard go under the stack of power plants, into a separate pile, or is it removed from the game?
They are discarded, which pretty much means they are removed from the game. The only time a power plant is put back under the stack of plants is during Phase 5: Update the power plant market.
Once a plant is discarded (because of too many plants) or removed from the game (no one bids on a plant in a turn, or someone's house count is higher than a power plant number), they aren't coming back, so "discarded" and "removed from the game" are pretty synonymous.
A «discarded» power plant is removed from the game.
When a player buys a fourth power plant, he must discard one of his other power plants. Power Grid Rules http://riograndegames.com/getFile.php?id=698