What distinguishes Mendelian Inheritance from Non-Mendelian Inheritance?
You can discuss multiple genes within the framework of Mendelian inheritance; what you're probably thinking of, though, is the fact that Mendelian inheritance doesn't recognize the idea of multiple genes that contribute to a single trait.

For example, if there is a gene that controls petal color (blue vs. white, with blue = dominant) and a gene that controls height (short vs. tall, with tall = dominant), then Mendelian inheritance predicts that two short plants with white flowers will only produce short plants with white flowers.

But if there are multiple genes that interact to determine height in a complex way, that's outside the scope of Mendelian inheritance.