The climax of this novel occurs when the men are hunting for Lennie after he has killed Curley's wife. As George and Lennie converse, there is a lot of tension because we if the men find Lennie they will kill him, and we also have the sense, and then know for certain, that George has brought a gun with him to find Lennie.
Steinbeck also uses the imagery of nature to let us know that Lennie's end is near.
A water snake glided smoothly up the pool, twisting its periscope head from side to side; and it swam the length of the pool and came to the legs of a motionless heron that stood in the shallows. A silent head and beak lanced down and plucked it out by the head, and the beak swallowed the little snake while its tail waved frantically. (Chap 6)
Steinbeck is letting us know that Lennie's death, much like the snake, is near. The tension building to the climas, is whether it will be by Curley's hand or George's.