Genesis 5:24 And Enoch walked with God: and he was not; for God took him. (KJV)
II Kings 2:11 And it came to pass, as they still went on, and talked, that, behold, there appeared a chariot of fire, and horses of fire, and parted them both asunder; and Elijah went up by a whirlwind into heaven. (KJV)
John 3:13 And no man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven. (KJV)
I Corinthians 15:50 Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption. (KJV)
First of all, the Genesis passage doesn't say that Enoch went to Heaven. Second of all, the 2 Kings passage doesn't say that Elijah went to Heaven, but to the sky itself. The Hebrew word is 2 Kings is 'shamayim' which means 'sky', not the place where God is. It should be pointed out, in fairness, that the word for 'heaven' used in John 3 (ouranos) can mean 'sky' or 'Heaven' (that is, the place where God is). In context, though, it's clearly referring to God's domain, not the sky. Not a contradiction.