Here's why:
* It represents a concept: It refers to the ability to survive in high temperatures.
* It can be the subject of a sentence: "Heat-survival is crucial for desert animals."
* It can be modified by adjectives: "Excellent heat-survival is essential for success in this environment."
While "heat" is an adjective and "survival" is a noun, their combination creates a new noun phrase that represents the idea of surviving heat.