I had some other idea to do this.
EmailTemplate template = [ SELECT Id, Name, Subject, Body, HtmlValue FROM EmailTemplate WHERE Name = 'My Name'];
templateId = template.Id;
Custom_Object__c obj = [ SELECT Name, Custom_Field_1__c, Custom_Field_2__c FROM Custom_Object__c WHERE Id = :my.Id];
Map data = new Map();
for ( String key : new List{'Name', 'Custom_Field_1__c', 'Custom_Field_2__c' } ) {
data.put('{!Custom_Object__c.' + key + '}', (String) obj.get(key) );
System.debug( LoggingLevel.ERROR,'@@@ key : ' + key );
System.debug( LoggingLevel.ERROR,'@@@ obj.get(key) : ' + obj.get(key) );
}
data.put('User.FirstName', UserInfo.getFirstName() );
data.put('User.LastName', UserInfo.getLastName() );
String body = template.HtmlValue;
System.debug( LoggingLevel.ERROR, body);
System.debug( LoggingLevel.ERROR,'@@@ template.HtmlValue: ' + template.HtmlValue );
System.debug( LoggingLevel.ERROR,'@@@ template.Subject: ' + template.Subject );
String subj = template.Subject;
for ( String key : data.keySet() ) {
body = body.replace( key, data.get( key ) );
subj = subj.replace( key, data.get( key ) );
}