Custom Posts, Custom Fields and Categories

You can use the function in_category( category #). Then use an if statement to show the custom field or not.

Oh i’m sorry i don’t understand what you mean.
Here is how i displayed my Categories:

<?php
$terms = wp_get_object_terms( $post->ID, 'category' );
  foreach( $terms as $term )
  $term_names[] = $term->name;
  echo implode( ', ', $term_names );
?>

I want to show the field Chapter: $chapter if the Category Novel is displayed in Categories.
How am i gonna do it? ??

You can use it like this in your single page template.

<?php
// used in the loop
// is this post in the category novel?
if ( in_category( 'novel', $post->ID ) )
{
        // if so, get the chapter meta and display it
	$chapter = get_post_meta($post->ID, 'chapter', true);
	echo "Chapter: ".$chapter;
}
?>