It all depends on how you define Heaven. 
 
As has been stated there are 3 separate Kingdoms of Glory that are referred to as heavens. So if you define heaven as these three kingdoms of Glory then it is more pertanint to ask who won't get in, not who will get in. The answer is only the sons of perdition will not obtain a kingdom of glory, ie. only the sons of perdition will not go to one of these heavens. Meaning that anyone who is not a son of perdition will go to heaven. 
If you define heaven as Exaltation then the answer is now. Only people who are baptized can get into the highest kingdom of Glory and only those sealed to their spouses can receive Exaltation. 
 
However, we do baptisms and sealings for the dead via proxy ordnances in the temple. Meaning if someone goes through life a non-mormon they will have the chance to become one in the next life. (a non-mormon is anyone who has not been baptized by God's authority). 
 
This means that ANYONE who lives and complies worthily with the law that they know and have recieved(ignorance is covered by christ), will have the chance to recieve baptism, and sealings and will have the chance to be exaulted. 
 
So even though some rice farmer who lived up to all the spirtual laws he knew, did not get baptized in this life, he will have the chance to recieve baptism by proxy and recieve exaltation. 
 
God is no respector of persons and wants everyone to have the chance to enter into his rest, if he sent us to a place where there is no hope of ever recieving the Gospel and then just dammed us to hell, how could he be just or merciful, and what would be the point.