Thoughts, comments..

20... Qf7 was perhaps a more powerful play on blacks part, it lacks in positional advantage some but would have slowed the promotion on c and had chances to continue the attack on whites kingside.

If 20... Qf7 then 21. Nd6 forks the King and Queen.

Well the opponent played badly, he had a lot better moves, but let's go ahead and say 19... Rxd5

Personally, I might have entertained the idea of 20... Qf4, it weakens the defence of the king, but ties the white queen down to the defence of the f-pawn as the knight is forced from its square. This may be a bit of a novice's view but perhaps there's enough time to cause white some trouble and ofset the threat of his c-pawn, although I must agree with gruppy, Rxd5 would have been vastly superior as it immediately removes the threat that ultimately wins the game in this case!

I think Qf4 would have been a better move.