How did Kanade die?
If Yuzuru's heart was donated to Kanade then why was she in the Afterlife before Yuzuru? Me and my friend were discussing this in class one day: we would reall want to know an answer or a fanfiction or anything.
Simply put, Otonashi wandered through many worlds, his memory gone. His soul finally came to rest in the afterlife school, but he did state that he wandered there.
We don't know how Kanade died, it can be assumed that something maybe went wrong with her transplant, or that she was just to weak after the transplant to live for much longer. Maybe thats why she wanted to thank him for his heart, because he gave her a little bit longer in life?
She was given more time thanks to Otonashi's heart. This is stated in Kanade's Background section.