Genesis 5:32 And Noah was five hundred years old: and Noah begat Shem, Ham, and Japheth. (KJV)
Genesis 7:6 And Noah was six hundred years old when the flood of waters was upon the earth. (KJV)
Genesis 11:10 These are the generations of Shem: Shem was an hundred years old, and begat Arphaxad two years after the flood: (KJV)
Note: The Assumption of contradiction is that Noah was 500 when he had Shem, then 600 when the flood happened. But two years later, Shem is only 100 when he would have been 102.
I could just dismiss this by saying that an approximation is not a contradiction. But that's actually not necessary here. Although Shem is listed first in Genesis 5:32, he is not the oldest child. The order listed there is not chronological according to birth. In fact, Genesis 9:24 says that Ham (the second listed) is the third child, and Genesis 10:21 suggests that Japheth (the third listed) is the first. So Noah was likely 500 when he had Japheth, then 502 when he had Shem, in which case Shem would have been 98 when the flood happened and then 100 when he had Arphaxad. Not a contradiction.