I never would have imagined that he would pass away so soon. I'm speechless.
I didn't imagine he would be gone so soon.
ご質問ありがとうございます。
このシチュエーションで「I didn't imagine」で文章を始めできます。そうすると、「I didn't imagine he would be gone so soon.」という風に言えます。
上記の文章はそのまま言えば、言いたいこと通じられるはずですが、もう少し自然な言い方を使いたい場合、「I never would have imagined that he would pass away so soon. I'm speechless.」と言えます。
因みに、「I'm speechless」という表現は「言葉を失う」という意味です。何か信じられないか有り得ないシチュエーションでよく使います。
ご参考になれば幸いです。
I would have never imagined that he would die so young.
I would have never thought that he would die so soon.
ーI would have never imagined that he would die so young.
「彼がそんなに若くして亡くなるとは想像してもみなかった」
I would have never imagined that he would ... で「彼が…とは想像してもみなかった」
ーI would have never thought that he would die so soon.
「彼がそんな早く亡くなるとは思ってもみなかった」
I would have never thought that he would ... で「彼が…とは思ってもみなかった」
ご参考まで！
アメリカ合衆国 
カナダ 
