After the US Presidential election, x candidate wins the Presidency. The candidate is then indicted by the FBI. Can the candidate be impeached by the House of Representatives and be prevented from taking office in the interim before the swearing in ceremony?
No, a President-elect is not President.
Article II, Section 4 of the Constitution states that:
"The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors."
It states that The President [...] of the United States, shall be removed from Office on Impeachment.
Since, a President-elect hasn't been sworn in, he/she is not technically considered a President. So, he/she can't be impeached.
Basically, you can't be fired from a job you just successfully applied for, since you haven't taken on the job yet.
As to whether the indicted candidate can take office:
Article II, Section 1 of the United States Constitution states:
No person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States.
The Twenty-second Amendment to the United States Constitution also states:
No person shall be elected to the office of the President more than twice, and no person who has held the office of President, or acted as President, for more than two years of a term to which some other person was elected President shall be elected to the office of the President more than once.
Since there's no mention of any other requirements. Even if he/she gets indicted, they can still take office.
Note that the date of the swearing in ceremony is set and there's no way changing it, unless a new law is passed.
But, if the President-elect is sworn in and takes office, it's then a totally different story.
Sidenote: This answer was first posted at U.S. president-elect impeachment trial before taking office?.