What is the meaning of "Like you wouldn't believe..." in the context?

I can't see what the underlined means below.
Would you show me about it?

-----The script goes :

[Scene: the hallway, Joey is moving in, Monica is leaving.]

Monica: Hi, again.

Joey: Hey! (goes into the apartment)

Chandler: (leaving to go to work) Hey!

Monica: Thank you soo, much.

Chandler: Oh, don’t thank me, thank the jerk that never showed up.
          Okay, I gotta get to get to work.

(Joey comes back into the hallway and starts to pick up a heavy box)

Monica: You want some help with that?

Joey: Oh, no thanks, I got it. (picks it up) No I don’t!

Monica: Whoa! Are you okay?

Joey: Whew! Stood up to fast, got a little head rush.

Monica: It’s the heat. (has her hand on his chest, and then pulls it away)
              And-and the humidity.

Joey: That’s a uh, that’s a tough combination.

Monica: Do you wanna come in for some lemonade?

Joey: Like you wouldn’t believe. (they go into the apartment) Wow!
      This is a great place.

Monica: Thank you. Just make yourself comfortable.

Joey: Gotcha.