[Buckley to Jack:] "I saved the Monopoly shoe and then it was gone. You took it. You act like she was only yours!" (18.60)
Buckley is about four when Susie dies, and, while he never completely understands what happened to her, he has a fair idea. When it seems clear to the family that she's met a bad end, Jack tries to explain the absence of the beloved sister to Buckley over a game of Monopoly. The Monopoly shoe was always Susie's piece. The removal of her piece from the game is what Jack uses to illustrate to Buckley her removal from the game of life, and he gives it to Buckley as a memento of his sister.
But, when it turns up missing it becomes, for Buckley, a symbol of what Susie's death has taken from him – namely, his father's love and attention throughout the years. In the difficult moment, Buckley begs him to let go of the dead and be there for him, the living. The very idea of stopping his continued vigil for Susie is so threatening to Jack that he has a heart attack.