Incomplete Game- Analysis Please?

This game was played a few days ago, and at the ending point my opponent resigned citing a decisive Black advantage.  I, however, do not think that Black is too awfully far ahead.  Would anyone be able to point out why White resigned this position?

Your fortress covers a lot of ground

Good bishops vs bad and okay knights

Penetrating rook and active queen vs semiactive queen and inactive rooks

Big pawn chain vs two active pawns

I would have played it out...

And won. Mwuhahaha

Computer analysis shows White is down two pawns - almost a minor piece, not to mention the complete lack of Tempo

After white retreats his knight, black can win a center pawn with his rook, leaving the pawn on c4 defenseless. This basically means black is about to be up two pawns and gain complete control of the center. I would have kept playing, but I have to say black has a good advantage.

In the final position white either loses the D-file or a knight.

A few sample variations to prove my point:

A) 18.Na7+? Kb8 and the knight is gone

B) 18.Na3?! Rxd3 19.Nb1 Bxc4 you would have won two pawns.

C) 18.Nc3?! Rxd3 and black wins the pawn on c4 soon.

D) 18.Nb3 Qb6 19.Na3 e4! Black has a big positional advantage

This is a rotten position and it seems that there is nothing wrong at resigning at this point.  Objectively speaking, black is winning.

I would keep playing anyways. There's always a possibility that black will blunder.