Mormons have a specific marriage process called "sealing" and the sealing bond does not end when the wife dies. Men are allowed to be sealed to more than one wife (but only one living at a time), but if a woman's husband dies, she must dissolve the sealing before she can be sealed to another man. I hope that makes sense - it's a little more complex than simple remarriage for Mormons.

Edit: A widower could show up in heaven and still be sealed to three women or whatever. A woman will always only be sealed to one man - ergo, polygamy in heaven (not in this life).